Vatican I speaks about the method of theology. Its words are few, but clear and precious. After pointing out that the mysteries of faith come to us only by God’s gift, the Council continues: “It is, nevertheless, true that if human reason, with faith as its guiding light, inquires earnestly, devoutly, and circumspectly, it does reach, by God’s generosity, some understanding of mysteries, and that a most profitable one. It achieves this by the similarity [analogia] with truths which it knows naturally and also from the interrelationship of mysteries with one another and with the final end of man” (DS 3016/1796). Even so, the mysteries are not grasped in the way that truths about the natural world are grasped. Faith remains necessary.

It is interesting to notice what the modest statement of Vatican I does not say. It does not say that rational reflection upon the mysteries of faith leads to a knowledge of truths through causes (Aristotle’s scientific method), nor that it establishes with certitude which propositions are truths of faith (the method of rationalist science), nor that it develops theories to account for the data of faith (the method of a modern science). Rather it says that reason can gain some understanding of mysteries by comparing one truth with another.

It seems to me that this teaching of Vatican I implies that the proper method for theology is the method called “dialectic” in Plato’s sense of the word. By this method, one considers truths of faith by comparison (analogia) with truths of reason, with one another, and with the ultimate fulfillment to which God calls us in the Lord Jesus. As is often said, one truth is considered in the light of another. To understand the dialectical method of theology, it is helpful to consider what is meant by “in the light of.”

If one considers the truth that Mary is the mother of Jesus in the light of the truth that Jesus is God, one sees the truth of Mary’s motherhood in a new light, for one knows her to be the mother of God. The relationship here is deductive. But nondeductive relationships among propositions also illuminate one truth by another. For example, propositions about revelatory words and deeds corresponding to one another are mutually illuminating. The truth that God made promises to Moses together with the truth that what was promised occurred communicate as neither by itself could do. The relationship in this case is not a deductive one. Again, the truths of the Old Testament illuminate those of the New and vice versa, since the gospel fulfills what God began with the law.

One not only finds the various truths of revelation mutually illuminating; one also comes to some understanding of faith in the light of truths naturally known by reason and vice versa. For example, the truth that human persons can make free choices pertains to faith (see DS 1555/815), but it also can be established by reason.⁴⁸ If this truth and the truth that humankind is made in God’s image are brought to bear upon the truth that God creates, the mystery of God’s creative act is not eliminated, but in some way it is illuminated, for it also is a matter of faith that God creates freely (see DS 3025/1805). Conversely, the truth about human aspirations for freedom and justice is understood in a different way by one who believes that human persons are made in God’s image than by a secular humanist.

Divine revelation, as personal communication, contains more than the propositional truths of faith. Tradition has an experiential aspect and also includes ways of living and worshipping whose reality the Church gradually comes to understand more and more perfectly. This development can be aided by theological reflection which brings into fruitful relationship truths of faith already implicitly taught by the Church and truths deeply imbedded in Christian imagery, experience, life, and worship. Thus, for instance, the truth that Jesus is truly a man is greatly illuminated by the practices and experiences related to the celebration of Christmas. This celebration gives us an irreducible awareness of the Incarnate Word’s solidarity with us in our human weakness and limitations. We knew the risen Lord of Easter when he was just a babe in arms.

In the ways exemplified and others, a dialectical method of meditation and discussion can lead to some understanding of mysteries of faith. This insight can provide a basis for genuine development of doctrine. Over and over again throughout history, questions are put to the Church which bear upon the realities of faith but which are formulated in concepts not previously in use among believers. For example, whether the human species could have evolved, using the word “evolved” in the precise sense in which it is used in Darwin’s Descent of Man, is a question which could not have been asked before Darwin.

Faced with such new questions, the Church must consider the possible answers in comparison with truths of faith already articulated, and judge which answer is in harmony with faith (see DS 3896/2327). This judgment is not an arbitrary one, since the Church must be faithful to what God has revealed. Moreover, not even the Church herself can change the meaning or contradict the truth of doctrines she already has taught as belonging to faith, since God’s revelation is present in the Church’s teaching (see DS 3020/1800).