Book III. In §§ 1-4, the words, I in the Father and the Father in Me, are taken as typical. Man cannot comprehend, but only apprehend them. So far as they are explicable Hilary explains them. But God's self revelation is always mysterious. The miracles of Christ are inexplicable (§§ 5-8); this is God's way, and meant to check presumption. Human wisdom is limited, and when it passes its bounds, and invades the realm of faith, it becomes folly. Next, in §§ 9-17, the passage, St. John xvii. 1 ff., is explained as proving that in the One God there are the Persons of Father and of Son, and as revealing God in the aspect of the Father. Then, in §§ 18-21, the wonderful deeds of Christ are put forth as an evidence of His wonderful birth. We must not ask how He can be coeternal with the Father, for it is in vain that we should ask how He could pass through the closed door. Either question is mere presumption. The revelation which Christ makes (§§ 22, 23) is that of God as His Father; Unum sunt, non Unus. And finally, in §§ 25, 26, he returns to the futility of reasoning. True wisdom is to believe where we cannot comprehend; we must trust to faith, not to proof.
Source: On the Trinity (New Advent)