1. Mankind's religious dimension
From the very beginning, mankind's history has had a religious dimension. The various expressions of this religiosity, once they are purified of the superstition that comes from ignorance and sin, show that the human person naturally has a conviction that there is a God who creates all things and on whom the world and our personal existence depend. Polytheism—the belief in many gods—has often appeared in human history, yet the deepest human religious experience and philosophical wisdom have sought the ultimate justification for the world and for human life in one, unique God. This God gives rise to all that exists and he is the fulfillment of our aspiration for happiness (cf. Catechism of the Catholic Church [CCC] 28). 
Despite the great diversity that we see among the arts, philosophy and literature of various cultures, all cultures come together in their reflection on God and on the central themes of human existence: life and death, good and evil, our ultimate end, the meaning of all things.  Since these manifestations of the human spirit have been present throughout history, we can say that reference to God belongs to human culture and constitutes an essential dimension of society and man. Hence, religious freedom is the first human right, and the search for God is the first human duty. All human beings are “impelled by nature and also bound by a moral obligation . . . to adhere to the truth, once it is known, and to order their whole lives in accord with the demands of truth."  The denial of God and the intent to exclude him from culture, society and political life are relatively recent phenomena, limited to some areas of the Western world. The fact that the great religious and existential questions remain unchanged over time  refutes the idea that religion belongs to an “infantile" phase of human history, destined to disappear with the progress of science.
Christianity takes up all that is good in the search for God and the worship of him throughout mankind's religious history. Moreover, Christianity gives to that history it true meaning: a path leading to the one true God who revealed himself in the history of salvation given to the people of Israel and who came to meet us by becoming man in Jesus Christ, the Incarnate Word. 
2. From material creatures to God: the cosmological proofs
The human intellect can know the existence of God, approaching him along paths that begin with the created world. There are two basic routes, one that passes through material creatures and one that passes through the human person. These paths or ways to God have been developed especially by Christian authors, but they have also been expounded by many philosophers and thinkers in many different times and cultures.
These ways that lead to the existence of God are called “proofs," but not in the sense that mathematics and natural science give to this term. Rather, they are called proofs because they are converging and convincing philosophical arguments that a person understands with greater or lesser depth depending on his or her specific formation (cf. CCC 31). That these proofs cannot be understood in the same way as the proofs used in experimental science is clearly seen from the fact that God is not an object of our empirical sense knowledge.
Depending on the particular philosophical approach used, each path to God's existence leads us to discover only one particular aspect or dimension of the absolute reality that is God. “Starting from movement, becoming, contingency and the world's order and beauty, one can come to a knowledge of God as the origin and the end of the universe" (CCC 32). The richness and incommensurability of God are such that no one of these ways by itself can reach a complete picture of God. Each reaches only a certain facet: his existence, his intelligence, his providence, etc.
Among the so-called cosmological ways, some of the best known are the famous “five ways" elaborated by St. Thomas Aquinas, who gathered together the philosophical reflections of the thinkers who had preceded him. In order to understand these ways one must be familiar with certain elements of metaphysics.  The first two ways are based on the idea of chains of causes that we observe in nature (seen in the movement from potency to actuality and in the movement from an efficient cause to its effect). Such chains of causes cannot go back infinitely but must arrive at a first mover and a first cause. The third way begins with the observation of contingency and limitation in natural beings and goes on to deduce that their cause must be a Being that is unconditioned and necessary. The fourth way focuses on the degrees of participated perfection that we find in things, and reaches the existence of a single infinite source of all these finite perfections. The fifth way observes the order and finality present in the world, a consequence of the stability of its laws, and goes on to reach the existence of an intelligence that orders all things and is their final cause.
Authors have set forth these and other analogous paths in diverse terms and in various forms right up to our own days. Consequently these ways remain applicable in our times. To understand these proofs, however, a person has to begin with a knowledge of things that is based on realism, and also reject any scientific reductionism that reduces our knowledge of reality to the findings of empirical science (and so avoid ontological reductionism as well); rather we must affirm that human thought can rise up from visible effects to invisible causes, as is affirmed by metaphysical thinking.
We can also reach knowledge of God through common sense, that is, through the spontaneous philosophical knowledge that is proper to every human being and arises from ordinary human experience: our wonder at the beauty and order of nature, our gratitude for the free gift of life, our grasp of the foundation of the goodness of things and of the reason for love. This type of knowledge is also important for grasping the subject to which the philosophical proofs for the existence of God refer. St. Thomas, for example, concludes each of his five ways with the same affirmation: “This is what everyone calls God," implying that we already have an idea of who God is.
Likewise, the testimony of Sacred Scripture (e.g., Wisdom 13:1-9; Romans 1:18-20; Acts 17: 22-27) and the teachings of the Church's Magisterium confirm that the human intellect, beginning from the knowledge of creatures, can reach knowledge of God the Creator (cf. CCC 36-38).  Also, both Scripture and the Magisterium point out that sin and twisted moral dispositions can make it more difficult to attain this knowledge.
3. The human spirit reveals God
Man recognizes his uniqueness and preeminence with regard to the rest of nature. Although biologically he has much in common with other species of animals, he also recognizes that in many crucial ways he is unique. He is able to reflect on himself and on his experience; he is capable of cultural and technological progress; he perceives the morality of his action. Because he possesses intellect and will, and above all because he enjoys freedom, he transcends the rest of the material cosmos.  Briefly put, the human person is the subject of a spiritual life that transcends matter, although he continues to depend upon matter (his body).  Right from the beginning, the culture and religious spirit of all peoples have explained this transcendence by affirming that the human person depends on God and that human life reflects God. Judeo-Christian revelation, in keeping with what reason shows, teaches that all men and women have been created in the image and likeness of God ( Gen 1:26-28).
Actually, the human person himself constitutes a path to God, for there are ways that lead to God beginning with one's own experience: “With his openness to truth and beauty, his sense of moral goodness, his freedom and the voice of his conscience, with his longings for the infinite and for happiness, man questions himself about God's existence. In all this he discerns signs of his spiritual soul" (CCC 33).
Our moral conscience approves the good and censures the evil we do or might want to do. It leads us to recognize a Supreme Good to which we must conform, and our conscience is, as it were, his messenger. Beginning with this experience of the human conscience—and even without knowing biblical revelation—many wise thinkers from ancient times reflected on the ethical dimension of human acts. In fact, every person is capable of such reflection insofar as he or she has been created in God's image.
In addition to his conscience, the human person is aware of his personal freedom as a condition for his moral acts. Because he is free, man realizes that he is responsible for his acts and that there exists someone to whom he is responsible. This Someone must be greater than any material thing and greater also than our fellow human beings, since they too, like us, are called to answer for their actions. Hence, the existence of human freedom and responsibility leads to the existence of a God, the guarantor of good and evil, the Creator, the Law-maker, and the one who rewards and punishes.
In our times the truth of human freedom is often denied. The human person is reduced to being little more than a highly developed animal, one whose acts are governed by necessary impulses. Some claim that the person's spiritual life (mind, conscience, soul) resides in the brain and in purely material neurophysiological processes which function according to necessary laws. Hence, they deny freedom and morality. These views can be countered with arguments based on reason and human phenomenology that show the transcendence of the person, the exercise of free-will even in choices conditioned by nature, and the impossibility of reducing the mind to the brain.
There are also many people who think that the presence of evil and injustice in the world are a proof for the non-existence of God, because if he did exist he would not allow evil. In reality even this uneasiness and this question are also “ways" to God. For we perceive evil and injustice as privations, as painful situations that ought not to exist, that clamor for a goodness and justice to which we aspire. If human beings, at the basic level of their natural structure, were not oriented toward the good, they would not be upset by the harm to the good and the privations that constitute evil.
The natural desires for the truth, the good, and happiness are signs of a natural human desire to see God. If such aspirations were frustrated, man would always be left unfulfilled, since these constitute the nucleus of a person's spiritual life and dignity. Their existence in the depths of our heart points to the existence of a Creator who calls us to himself through hope in him. If the “cosmological" ways do not give assurance of the possibility of reaching God as a personal being, the “anthropological" ways that begin with man and his natural desires allow us to see that the God we depend upon must be a person capable of love, a personal being who cares for his creatures.
Sacred Scripture contains explicit teachings about the existence of a moral law written by God in the human heart (cf Wis 15:11-20; Ps 19; Rom . 2:12-16). Philosophy that is inspired by Christianity calls this the “natural moral law." It states that this law is accessible to all men of all times and cultures, even though it also admits that the recognition of truths such as the existence of God can be obscured by sin. The Church's Magisterium has repeatedly underscored the existence of conscience and freedom as ways to God. 
4. The denial of God: the causes of atheism.
The different philosophical arguments presented to “prove" the existence of God do not, by themselves, produce faith in (the revealed) God. Taken alone, they only show that such a faith is reasonable. There are several reasons why a person, despite these proofs, might not come to have faith in the God of revelation. First of all, these arguments, while they show that God exists and lead a person to consider philosophically some of his characteristics (his goodness, intelligence, etc.), do not tell us who the personal being is to whom one directs the act of faith. Second, faith is a free response of man to God who reveals himself and not a necessary philosophical deduction; despite proofs, a person may withhold the assent of faith. Third, it is primarily God and not the person himself who is the cause of faith. God reveals himself gratuitously and with his grace moves man's heart to adhere to him. Fourth, we need to recognize that sin has obscured man's mind and wrapped it in darkness, making difficult the recognition of God's existence and a response of faith to his Word (cf. CCC 37). For all these reasons, particularly the last, it is always possible for a person to deny God. 
Atheism takes on two forms. The theoretical form tries positively to deny the existence of God on the basis of reason. The practical form, on the other hand, effectively denies God by living as if he did not exist. To profess positive, theoretical atheism as a consequence of a rational analysis based on empirical science is contradictory, because God, as a spiritual being, is not the object of experimental scientific knowledge. It is possible to deny the existence of God on the basis of philosophical reasoning, but only when one begins with a preconceived view of reality, as happens with materialism and other ideologies. That such philosophical positions lack a sound basis can always be shown with the help of metaphysics and a theory of knowledge that is based on reality.
Positive atheism often results from the notion that to accept God implies a limitation on man: if God exists, then we are not free and we do not enjoy full autonomy here on earth. This way of looking at things fails to consider that a creature's freedom and autonomy flow from that creature's dependence on God.  In fact, the truth is just the opposite. As history has shown, particularly in our times, when one denies God one ends up denying man and his transcendent dignity as well.
Others deny God because they think that religion, especially Christianity, is the fruit of ignorance and superstition and that it represents an obstacle to human progress. We can respond to this objection on historical grounds, for it is possible to show the positive influence of Christian revelation on the concept of the human person and human rights, and also on the origin and progress of science. The Catholic Church in fact has always considered ignorance (rightly) as an obstacle to the true faith. In general, those who deny God to affirm the perfection and the progress of humanity do so in order to defend a this-worldly view of historical progress that denies man's fulfillment in the world to come. The ends they propose are a political utopia or a purely material well-being incapable of fully satisfying the deeper desires of the human heart.
Among the causes of atheism, especially practical atheism, we must also include the bad example of believers, who “to the extent that they neglect their own training in the faith, or teach erroneous doctrine, or are deficient in their religious, moral or social life … must be said to conceal rather then to reveal the authentic face of God and religion."  On the other hand, beginning with Vatican II the Church has always pointed to the testimony of Christians as the principal factor in carrying out the “new evangelization." 
5. Agnosticism and religious indifferentism
Agnosticism, which is especially widespread in intellectual circles, teaches that human reason cannot conclude anything about God and his existence. Its defenders frequently put great effort into their personal and social life, but they do so without any reference to a last end. Thus they seek to live a humanism without God. This agnostic position frequently ends up merging with practical atheism, since a person who lives his daily life and chooses his partial ends without any commitment to the (one) ultimate end to which he naturally tends has in fact actually decided on an end for his life: a selfish, this-worldly end. The position taken by agnostics may merit some respect, but its supporters should be helped to show the honesty of their non-denial of God by remaining open also to the possibility of admitting his existence and the historical fact of revelation.
Today religious indifferentism is the principle manifestation of unbelief, and as such it has received increased attention from the Church's Magisterium.  The topic of God is not taken seriously or simply ignored because in practice it is suffocated by a life devoted to material goods. Religious indifferentism coexists with a certain empathy for the sacred and even for the pseudo-religious, but without any moral considerations, as if they were simply consumer goods. Maintaining a position of religious indifferentism for a long time requires continuous distractions in order to avoid facing the more important problems in life. It takes an effort to keep questions like the meaning of life and death or the moral value of one's actions out of one's daily life and out of one's conscience. But in every person's life there come “pivotal" events (falling in love, becoming a father or a mother, unexpected deaths, joys and sorrows, etc.) and one simply cannot maintain a position of religious “indifferentism" throughout life. At some point all men and women have to ask themselves about God. When these significant events occur in peoples' lives, one needs to help them to take seriously the search for God and the reality of his existence.
6. Religious pluralism: there is only one true God and he has revealed himself in Jesus Christ.
The religious spirit in man, which when it is authentic leads to recognizing the one true God, has manifested itself and found expression in the history and in the culture of many peoples in different ways. It has manifested itself in diverse ideas about the divinity and in the worship of different images. The world religions that reveal man's sincere quest for God and value the transcendent dignity of man must be respected. The Catholic Church considers that there is present in them a “spark" or a participation in the divine Truth.  In studying the various religions, however, proper distinctions must be made. One must recognize the presence of superstitions and ignorance, of forms of irrationality, and of practices not in accord with the freedom and dignity of the human person.
Inter-religious dialogue is not opposed to evangelization. In addition, while respecting the freedom of each individual, the purpose of dialogue must always be that of announcing Christ. The seeds of truth that can be found in non-Christian religions are in fact seeds of the One Truth that is Christ. Consequently, these religions have the right to receive God's revelation and to be brought to maturity through the knowledge of Christ who is the Way, the Truth and the Life. God does not deny salvation to those who without guilt do not know the Gospel but yet live according to the natural moral law and thus show that they consider that law to be based on the one true God. 
In this inter-religious dialogue Christianity can make clear that the world religions, when they are authentic expressions of our link to the one true God, reach their fulfillment in Christianity itself. Only in Christ does God reveal man to himself, offering the solution to the mysteries of his existence and unveiling the deepest meaning of his aspirations. Christ is the sole mediator between God and mankind. 
Christians can approach inter-religious dialogue with hope and optimism, because they know that every man and woman has been created in the image of the one true God. Every person who reflects in the silence of his heart, can listen to the witness of his own conscience, which will lead him to the one God revealed in Jesus Christ. “This is why I was born, and why I have come into the world," Christ said to Pilate, “to bear witness to the truth. Everyone who is of the truth, hears my voice" ( Jn 18:37). In this sense Christians can speak about God without risk of being intolerant, because the God about whom they speak, the God that can be recognized in nature and in the conscience of each individual, the God who created the heavens and the earth, is the God of the history of the salvation who revealed himself to Israel and who became man in Jesus Christ. This is the path followed by the first Christians. They refused to adore Christ as one more among the gods of the Roman pantheon, because they were convinced of the existence of the one true God. At the same time they were bent on showing that the God glimpsed by the philosophers as cause, reason and foundation of the world was and is the very God of Jesus Christ. 
Catechism of the Catholic Church, 27-49.
Vatican II, Gaudium et spes , 4-22.
John Paul II, Fides et ratio , 14 September1998, 16-35.
Benedict XVI, Spe salvi , 30 November 2007, 4-12.
 Cf John Paul II, Enc. Fides et ratio , 14 September 1998, 1.
 “Transcending all the differences which distinguish individuals and peoples, there is a fundamental commonality. For different cultures are but different ways of facing the question of the meaning of personal existence. And it is precisely here that we find one source of the respect, which is due to every culture and every nation: every culture is an effort to ponder the mystery of the world and in particular of the human person: it is a way of giving expression to the transcendent dimension of human life. The heart of every culture is its approach to the greatest of all mysteries: the mystery of God." John Paul II, Address to the United Nations, New York, 5 October 1995, no. 9.
 Cf. Vatican II, Dignitatis humanae , 2.
 Cf. Vatican II, Gaudium et spes , 10.
 Cf. John Paul II, Tertio millennio adveniente , 10 November 1994, 6; Enc. Fides et ratio , 2.
 Cf. St. Thomas Aquinas , Summa Theologiae , I, q. 2, a. 3 ; Summa Contra Gentiles , I, c. 13. A more detailed explanation can be found in these two references as well as in other manuals of metaphysics and natural theology.
 Cf. Vatican I, Dei Filius ; Motu Proprio Sacrorum Antistitum (1910); Congregation for the Doctrine of the Faith, Donum veritatis (1990), 10; John Paul II, Fides et ratio , 67.
 “We have learned with gratitude, because it makes us realize the happiness we are being called to, that all creatures have been created out of nothing by God and for God: both men, who are rational creatures, although we so often act unreasonably, and the irrational beings who roam the surface of the earth, or burrow in its inmost recesses, or sail the azure skies – some soaring so high that they come face to face with the sun. But in all this wonderful variety, it is only we men (I am not referring now to the angels) who can unite ourselves to the Creator by using our freedom. We are in a position to give him, or deny him, the glory that is his due as the Author of everything that exists." St. Josemaria, Friends of God , 24.
 Cf. Vatican II, Gaudium et spes , 18.
 Ibidem, 17-18. In particular, John Paul II has amply developed the doctrine on moral conscience and the responsibility linked to human freedom, while stressing that the human person is created in the image of God. Cf. Veritatis splendor (1993), 54-64.
 See Vatican II, Gaudium et spes , 19-21.
 Ibidem, 36.
 Ibidem, 19.
 Ibidem, 21; Paul VI, Evangelii nuntiandi , 21; John Paul II, Veritatis splendor , 93; John Paul II, Novo millennio ineunte , chs. III and IV.
 Cf. John Paul II, Christifideles laici , 34; Fides et ratio , 5.
 Cf. Vatican II, Nostra aetate , 2.
 Cf. Vatican II, Lumen gentium , 16.
 Cf. John Paul II, Redemptoris missio , 5; Congregation for the Doctrine of the Faith, Dominus Iesus (2000), 5, 13-15.
 Cf. John Paul II, Fides et ratio , 34; Benedict XVI, Spe Salvi , 5.