16 By assuming flesh, however, He acquired our nature in our totality, and became all that we are, but did not lose that which He was before. Both before by His heavenly origin, and now by His earthly constitution, God is His Father. By His earthly constitution God is His Father, since all things are from God the Father, and God is Father to all things, since from Him and in Him are all things. But to the Only-begotten God, God is Father, not only because the Word became flesh; His Fatherhood extends also to Him Who was, as God the Word, with God in the beginning.
Thus, when the Word became flesh, God was His Father both by the birth of God the Word, and by the constitution of His flesh: for God is the Father of all flesh, though not in the same way that He is Father to God the Word. But God the Word, though He did not cease to be God, really did become flesh: and while He thus dwelt He was still truly the Word, just as when the Word became flesh He was still truly God as well as man. For to 'dwell' can only be said of one who abides in something: and to become flesh of one who is born.
He dwelt among us; that is, He assumed our flesh. The Word became flesh and dwelt among us; that is, He was God in the reality of our body. If Christ Jesus, the man according to the flesh, robbed God the Word of the divine nature, or was not according to the mystery of godliness also God the Word, then it reduces His nature to our level that God is His Father, and our Father, His God and our God. But if God the Word, when He became the man Christ Jesus, did not cease to be God the Word, then God is at the same time His Father and ours, His God and ours, only in respect of that nature, by which the Word is our brother, and the message to His brethren, I ascend unto My Father and your Father, and My God and your God, is not that of the Only-begotten God the Word, but of the Word made flesh.
Source: On the Trinity (New Advent)