Heretics endeavouring to get over the words, I and My Father are one, as a proving unity of nature, and to reduce them to mean a unity simply of natural love, and agreement of will, bring forwards these words of our Lord’s as an example of this kind of unity: That they may be all one, as Thou, Father, art in Me, and I in Thee. But though impiety can cheat its own understanding, it cannot alter the meaning1 of the words themselves. For they who are born again of a nature that gives unity in life eternal, they cease to be one in will merely, acquiring the same nature by their regeneration: but the Father and Son alone are properly one, because God, only-begotten of God, can only exist in that nature from which He is derived.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: viii. de Trin