Every Christian believes that God exists.
Thus, every Christian believes not only that God has spoken and that He assumed human flesh in a historical figure at the time of the Roman Empire: Jesus of Nazareth.
But a Catholic goes beyond this, believing that God and Christ live and act-as in a "body," to use a term from the New Testament-in that Church, the visible leader of which, on earth, is the Bishop of Rome.
Faith, certainly, is a gift, a divine grace. But another divine gift is reason. According to the ancient exhortations of the saints and doctors of the Church, the Christian "believes in order to understand"; but he is also called "to understand in order to believe."
Let's start, then, at the beginning. Your Holiness, from a human perspective, can (and how can) one come to the conclusion that God really exists?
Your question ultimately concerns Pascal's distinction between the
Absolute-that is, the God of the philosophers (the rationalist
libertins)-and the God of Jesus Christ; and, prior to Him,
the God of the Patriarchs-from Abraham to Moses. Only the God of Jesus
Christ is the living God. As has also been stated in the Dogmatic
Constitution on Divine Revelation Dei Verbum (no. 3), the first God
mentioned above-the God of the philosophers-is the fruit of human thought,
of human speculation, and capable of saying something valid about God. In
the end, all rationalist arguments follow the path indicated in the Book of
Wisdom and the Letter to the Romans-passing from the visible world to the
Aristotle and Plato follow this same path, but in a different manner. The Christian tradition before Thomas Aquinas, and therefore also Augustine, was tied to Plato, from whom it nonetheless rightfully wanted to distance itself. For Christians, the philosophical Absolute, considered as the First Being or Supreme Good, did not have great meaning. Why engage in philosophical speculations about God, they asked themselves, if the living God has spoken, not only by way of the Prophets but also through His own Son? The theology of the Fathers, especially in the East, broke away more and more from Plato and from philosophers in general. Philosophy itself, in the Fathers, ends up in theology (as in the case, for example, in modern times, of Vladimir Soloviev).
Saint Thomas, however, did not abandon the philosophers' approach. He began his Summa Theologica with the question "An Deus sit?"-"Does God exist?" (cf. 1, q.2, a.3). You ask the same question. This question has proven to be very useful. Not only did it create theodicy, but this question has reverberated throughout a highly developed Western civilization. Even if today, unfortunately, the Summa Theologica has been somewhat neglected, its initial question persists and continues to resound throughout our civilization.
At this point it is necessary to cite an entire passage from the Pastoral Constitution Gaudium et Spes of the Second Vatican Council: "In truth, the imbalances existing in the modern world are linked to a more profound imbalance found in the heart of man. Many elements conflict with each other in man's inner struggle. As a created being, he experiences his limitations in thousands of ways yet he also perceives himself to be boundless in his aspirations and destined to a higher life. Enticed by many options, he is continually forced to choose some and to renounce others. Furthermore, since he is weak and sinful, he often does what he detests and not what he desires. This causes him to suffer an inner division, which is the source of so many and such grievous disagreements in society. . . . With all of this, however, in face of the modern world's development, there is an ever-increasing number of people who ask themselves or who feel more keenly the most essential questions: What is man? What is the meaning of suffering, of evil, of death, which persist despite all progress?What are these victories, purchased at so high a cost, really worth?What can man offer to society and what can he expect from it? What will there be after this life?The Church believes that Christ, who died and was resurrected for the sake of all, continuously gives to man through His Spirit the light and the strength to respond to his higher destiny. Nor is there any other name under heaven given to the human race by which we are to be saved. The Church also believes that the key, the center, and the purpose of all of human history, is found in its Lord and Master" (Gaudium et Spes 10).
This passage of the Council is immensely rich. One clearly sees that the response to the question "An Deus sit?" is not only an issue that touches the intellect; it is, at the same time, an issue that has a strong impact on all of human existence. It depends on a multitude of situations in which man searches for the significance and the meaning of his own existence. Questioning God's existence is intimately united with the purpose of human existence. Not only is it a question of intellect; it is also a question of the will, even a question of the human heart (the raisons du coeur of Blaise Pascal). I think that it is wrong to maintain that Saint Thomas's position stands up only in the realm of the rational. One must, it is true, applaud Etienne Gilson when he agrees with Saint Thomas that the intellect is the most marvelous of God's creations, but that does not mean that we must give in to a unilateral rationalism. Saint Thomas celebrates all the richness and complexity of each created being, and especially of the human being. It is not good that his thought has been set aside in the post-conciliar period; he continues, in fact, to be the master of philosophical and theological universalism. In this context, his quinque viae-that is, his "five ways" that lead toward a response to the question "An Deus sit?"-should be read.