44 And the persecutor, who was converted to be an Apostle and a chosen vessel, delivers the very same message. What discourse is there of his which does not presuppose the confession of the Son? What Epistle of his that does not begin with a confession of that mysterious truth? When he says, We were reconciled to God by the death of His Son, and, God sent His Son to be the likeness of the flesh of sin, and again, God is faithful, by Whom you were called unto the fellowship of His Son, is any loophole left for heretical misrepresentation? His Son, Son of God; so we read, but nothing is said of His adoption, or of God's creature.
The name expresses the nature; He is God's Son, and therefore the Sonship is true. The Apostle's confession asserts the genuineness of the relation. I see not how the Divine nature of the Son could have been more completely stated. That Chosen Vessel has proclaimed in no weak or wavering voice that Christ is the Son of Him Who, as we believe, is the Father. The Teacher of the Gentiles, the Apostle of Christ, has left us no uncertainty, no opening for error in his presentation of the doctrine.
He is quite clear upon the subject of children by adoption; of those who by faith attain so to be and so to be named. in his own words, For as many as are led by the Spirit of God, they are the sons of God. For you have not received the spirit of bondage again unto fear, but you have received the Spirit of adoption, whereby we cry, Abba, Father. This is the name granted to us, who believe, through the sacrament of regeneration; our confession of the faith wins us this adoption.
For our work done in obedience to the Spirit of God gives us the title of sons of God. Abba, Father, is the cry which we raise, not the expression of our essential nature. For that essential nature of ours is untouched by that tribute of the voice. It is one thing for God to be addressed as Father; another thing for Him to be the Father of His Son.
Source: On the Trinity (New Advent)