It was necessary for our salvation that there be a knowledge revealed by God, besides philosophical science built up by human reason. Firstly, indeed, because the human being is directed to God, as to an end that surpasses the grasp of his reason. "The eye hath not seen, O God, besides Thee, what things Thou hast prepared for them that wait for Thee" (Isaiah 64:4). But the end must first be known by men who are to direct their thoughts and actions to the end. Hence it was necessary for the salvation of man that certain truths which exceed human reason should be made known to him by divine revelation.

Part I, Question 1, Article 1; tr. Fathers of the English Dominican Province (1920, New York: Benziger Bros.) - Summa Theologica (1265–1274)