4 We, on the contrary, having by spiritual teaching arrived at the faith of the evangelists and apostles, and following after the hope of eternal blessedness by our confession of the Father and the Son, and having proved out of the Law the mystery of God and God, without overstepping the limits of our faith in one God, or failing to proclaim that Christ is God, have adopted this method of reply from the Gospels, that we declare the true nativity of Only-begotten God from God the Father, because that through this He was both very God and not alien from the nature of the One very God, and thus neither could His Godhead be denied nor Himself be described as another God, because while the birth made Him God, the nature within him of one God of God did not separate Him off as another God.
And although our human reason led us to this conclusion, that the names of distinct natures could not meet together in the same nature, and not be one, where the essence of each did not differ in kind; nevertheless, it seemed good that we should prove this from the express sayings of our Lord, Who after frequently making known that the God of our faith and hope was One, in order to affirm the mystery of the One God, while declaring and proving His own Godhead, said, I and the Father are one; and, If you had known Me, you would have known My Father also; and, He that has seen Me has seen the Father also; and, Believe Me, that the Father is in Me, and I in the Father: or else believe for the very works' sake. He has signified His own birth in the name Father, and declares that in the knowledge of Himself the Father is known.
He avows the unity of nature, when those who see Him see the Father. He bears witness that He is indivisible from the Father, when He dwells in the Father Who dwells in Him. He possesses the confidence of self-knowledge when He demands credit for His words from the operations of His power. And thus in this most blessed faith of the perfect birth, every error, as well that of two Gods as of a single God, is abolished, since They Who are one in essence are not one person, and He Who is not one person with Him Who is, is yet so free from difference from Him that They Two are One God.
Source: On the Trinity (New Advent)