18 So we have made our reply to the folly of our violent opponents, merely to prove the emptiness of their falsehoods and so prevent them from misleading the unwary by the error of their vain and foolish statements. But the faith of the Gospel did not of necessity require our answer. The Lord prayed on our behalf for our union with God, but God keeps His own unity and abides in it. It is not through any mysterious appointment of God that they are one, but through a birth of nature, for God loses nothing in begetting Him from Himself. They are one, for the things which are not plucked out of His hand are not plucked out of the hand of the Father, for, when He is known, the Father is known, for, when He is seen, the Father is seen, for what He speaks the Father speaks as abiding in Him, for in His works the Father works, for He is in the Father and the Father in Him. This proceeds from no creation but from birth; it is not brought about by will but by power; it is no agreement of mind that speaks, it is nature; because to be created and to be born are not one and the same, any more than to will and to be able; neither is it the same thing to agree and to abide.
Source: On the Trinity (New Advent)