Before the incomprehensible depth of the Christian mystery and given the endless variety of men who want to get the call of Christianity, it is impossible to tell all at once something about the concept of it. An introduction to the mystery may be too "high", complicated and abstract for some and too elementary for others.

However, from a simple faith of catechism, on the one hand, and the study of all sciences related to religion, on the other, there is a way to justify Christian intellectual honesty, for man does not live the whole of its existence and the broad dimensions of the same individual according to the findings of contemporary science and all, however, is accountable to his intellectual conscience of the whole of its existence indirectly and primary.

Karl Rahner rush this perspective the company to express the whole of Christianity and take responsibility for it. Foundations of faith is certainly the most elaborate work of one of the most influential Catholic theologians of the twentieth century.