Compendium, 1, 1, 1: "Man's Capacity for God" (3-4)

3. How is it possible to know God with only the light of human reason?

(Catechism 31-36, 46-47)
Starting from creation, that is from the world and from the human person, through reason alone one can know God with certainty as the origin and end of the universe, as the highest good and as infinite truth and beauty.

4. Is the light of reason alone sufficient to know the mystery of God?

(Catechism 37-38)
In coming to a knowledge of God by the light of reason alone man experiences many difficulties. Indeed, on his own he is unable to enter into the intimacy of the divine mystery. This is why he stands in need of being enlightened by God’s revelation, not only about those things that exceed his understanding, but also about those religious and moral truths which of themselves are not beyond the grasp of human reason, so that even in the present condition of the human race, they can be known by all with ease, with firm certainty and with no admixture of error.

The certainty spoken of here does not mean scientific certainty - there are no proofs of God that can be verified in a laboratory. The certainty spoken of here is a reliance on man's reason, and the assertion that faith is reasonable. This principle has been key in the ministry of Pope Benedict XVI, that faith and reason are not incompatible, but rather are informed by one another. The Catechism says, " 'Our holy mother, the Church, holds and teaches that God, the first principle and last end of all things, can be known with certainty from the created world by the natural light of human reason.' Without this capacity, man would not be able to welcome God's revelation. Man has this capacity because he is created 'in the image of God' " (34).

Man can know God, as St Augustine puts it, by questioning nature. But that does not mean that the road to faith is without obstacle, or that all men will come to know God through the study of nature. There are many, many obstacles for each of us to come to know God, to share in his life. Thanks be to God, however, that he continually and faithfully draws all men to himself regardless of the difficulties.