John Chrysostom

Why does he not say, "For as many of you as have been baptized into Christ, have been born of God?" for this was what directly went to prove that they were sons;-because he states it in a much more awful point of view; If Christ be the Son of God, and thou hast put on Him, thou who hast the Son within thee, and art fashioned after His pattern, hast been brought into one kindred and nature with Him.

Homily on Galatians 3 ↗

Theophylact of Ohrid

Defining in what way we are sons of God, he says: through baptism. But he did not say: you who were baptized became children of God, as the sequence would seem to require, — but far more expressively: you have put on Christ. And if we have put on Christ, the Son of God, and have been made like unto Him, it means we have been brought into one kinship, into one image, having become by grace what He is by nature.

Commentary on Galatians ↗

Oecumenius

Since he called us children of God in a splendid way, he also speaks of the manner in which we became so. But it is fitting to say regarding the preceding sense, "All who were baptized into Christ have become children of God"; for this is the consequence. Yet now he has said the same thing in another way, more appropriately expressing it. For if we have put on the Son of God, and as it were have clothed ourselves with his image, it is clear that we are also sharers in his sonship. …

Commentary on Galatians ↗

Jerome

For whoever has been baptized in Christ has clothed themselves with Christ. There is neither Jew nor Greek, slave nor free, male nor female. For you are all one in Christ Jesus. And how we are born as children of God through faith in Christ Jesus, he demonstrates by saying: For whoever has been baptized in Christ has clothed themselves with Christ. And that Christ is our clothing is proven not only in this passage, but also in another by the urging of Paul himself: Clothe yourselves with the Lord Jesus Christ (Rom. …

Commentary on Galatians ↗

Thomas Aquinas

Then when he says, "For as many of you as have been baptized in Christ have put on Christ," he expounds his proposition. Concerning this he does three things: First, he proposes to explain the proposition; Secondly, the elucidation of the explanation (v. 28); Thirdly, he assigns the reason behind the explanation (v. 28): "For you are all one in Christ Jesus." With respect to the first, he shows how we are sons of God in Christ Jesus. …

Commentary on Galatians ↗