The Church teaches that one can know with certainty of the existence of God by the natural light of human reason. This statement is surprising to many Catholics and non-Catholics alike. Aquinas is an excellent source, starting with “Summa contra Gentiles”. William Lane Craig, though not Catholic, speaks a great deal about this, check out his videos. And check John Paul II’s “Fides et Ratio”.
EDIT: Somebody else mentioned Fr. Spitzer, and I feel stupid for not including him. Definitely another good resource.