A brief definition of “faith,” according to Ratzinger, in “Introduction to Christianity”

Cardinal Ratzinger teaches that faith is what creates meaning to human existence. The faith of a Christian is embodied in the Apostle’s Creed, which begins with the words “I believe,” and ends with the word “Amen.” Ratzinger argues that there is a spiritual, “invisible” world that is just as real—indeed more real—than the material world. We cannot, however, see or “lay[] it on the table” (70), but while we can’t prove the object of our faith objectively, this does not diminish its truth or importance.

Ratzinger sees faith and reason as allies, but acknowledges that the “foothold” from which we find true understanding and meaning in life is the materially unverifiable acknowledgment of God revealed, through Christ, redeeming mankind. Using Isaiah’s concept of faith as a “foothold,” Ratzinger argues that with all the success of human calculation, and with all we have learned in measuring the world, without faith, we cannot measure ourselves.

Opposing this empty materialism is at the heart of the Church’s teaching that man cannot live on bread alone. In this way, Ratzinger points out that while it is difficult to entrust “ourselves to what cannot be seen,” the invisible has a primacy over the visible. The proof of this can be seen in the misery of nihilism, and the fact that today so many living in such comfort “fall[] into the situation of no longer being able to live” (73). In sum, the Hebrew version of Isaiah 7:9 (if you don’t believe, you have no foothold) is perfectly reconcilable with the Ancient Greek interpretation (if you don’t believe then you don’t understand). Faith, then, to Ratzinger, is standing firm on the ground “of the word of God” (69), giving purpose to those who believe.