46 According to the teaching of the Catholic Church, the Christian mind must first be nourished in simple faith, in order that it may become capable of understanding things heavenly and eternal. Thus it is said by the prophet: "Unless ye believe, you shall not understand." Simple faith is that by which, before we attain to the height of the knowledge of the love of Christ, that we may be filled with all the fullness of God, we believe that not without reason was the dispensation of Christ's humiliation, in which He was born and suffered as man, foretold so long before by the prophets through a prophetic race, a prophetic people, a prophetic kingdom.
This faith teaches us, that in the foolishness which is wiser than men, and in the weakness which is stronger than men, is contained the hidden means of our justification and glorification. There are hid all the treasures of wisdom and knowledge, which are opened to no one who despises the nourishment transmitted through the breast of his mother that is, the milk of apostolic and prophetic instruction; or who, thinking himself too old for infantile nourishment, devours heretical poison instead of the food of wisdom, for which he rashly thought himself prepared.
To require simple faith is quite consistent with requiring faith in the prophets. The very use of simple faith is to believe the prophets at the outset, while the understanding of the person who speaks in the prophets is attained after the mind has been purified and strengthened.
Source: Reply to Faustus the Manichaean (New Advent)