7 “I have many things,” He said, “to say of you and to judge: but He that sent me is true;” as if He had said, Therefore I judge the truth, because, as the Son of the True One, I am the truth. The Father true, the Son the truth—which do we account the greater? Let us reflect, if we can, which is the greater, the True One or the Truth. Take some other instances. Is a pious man, or piety, the more comprehensive? Surely piety itself; for the pious is derived from piety, not piety from the pious.
For piety may still exist, though he who was pious became impious. He has lost his piety, but has taken nothing from piety itself. What also of comely and comeliness? Comeliness is more than comely; for comeliness gives existence to the comely, not the comely to comeliness. And so of chaste and chastity. Chastity is clearly something more than chaste. For if chastity had no existence, one would have no ground to be chaste; but though one may refuse to be chaste, chastity remains entire.
If then the term piety implies more than the term pious, comeliness more than comely, chastity than chaste, shall we say that the Truth is more than the True One? If we say so, we shall begin to say that the Son is greater than the Father. For the Lord Himself says most distinctly, “I am the way, and the truth, and the life.” Therefore, if the Son is the truth, what is the Father but what the Truth Himself says, “He that sent me is true”? The Son is the truth, the Father true. I inquire which is the greater, but find equality. For the true Father is true not because He contained a part of that truth, but because He begot it entire.
Source: Tractates on the Gospel of John (New Advent)