That God is one and three is, of course, for Aquinas a profound mystery which we could not hope to know apart from divine revelation, but we can only begin to understand what he has to say about it if we recognise that for him God is a profound mystery anyway. There are people who think that the notion of God is a relatively clear one; you know where you are when you are simply talking about God whereas when it comes to the Trinity we move into the incomprehensible where our reason breaks down. To understand Aquinas it is essential to see that for him our reason has already broken down when we talk of God at all—at least it has broken down in the sense of recognising what is beyond it. Dealing with God is trying to talk of what we cannot talk of, trying to think of what we cannot think. Which is not to say that it involves nonsense or contradiction.

This similarity is sometimes obscured for us by the fact that Aquinas thinks we can prove the existence of God by natural reason whereas such unaided natural reason could tell us nothing of the Trinity. This, however, does not, for him, make the latter a mystery where the former is not, for he thought that to prove the existence of God was not to understand God but simply to prove the existence of a mystery. His arguments for the existence of God are arguments to show that there are real questions to which we do not and cannot know the answer.