John Chrysostom

What can the heretics say to this? for as, according to them, the expression "the Only True God" excludes the Son from being true God, so here the phrase "God is One," excludes Him from being God in any sense. But if, although the Father is called "One God," the Son is nevertheless God, it is very plain that though the Father is called "Very God," the Son is very God likewise. Now a mediator, says he, is between two parties; of whom then is Christ the Mediator? plainly of God and of men. …

Homily on Galatians 3 ↗

Theophylact of Ohrid

So also Christ serves as a mediator between two parties, namely, between God and men. For He reconciled both, giving peace and destroying the enmity that men had with God. Since He united human nature with Himself, He gave peace, wondrously uniting with the Divine nature the flesh that was hostile to it on account of sin. Therefore, if He is the mediator and reconciler, then it is clear that He saves, and not the law.

Commentary on Galatians ↗

Oecumenius

"Now a mediator is not of one, but God is one." The mediator, he says, is the mediator of two. Of whom then is Christ the mediator? Clearly of both men and God. Do you see how he shows that he both gave the law himself, and would also be Master and to loosen it again? "Now a mediator is not of one." He says therefore that Moses indeed mediated for the giving of the law, but he mediated for only one nation. …

Commentary on Galatians ↗

Thomas Aquinas

Then when he says, "Now a mediator is not of one," he explains what he meant when he said, "in the hand of a mediator." This can be explained in three ways. In one way, that a mediator is not of one alone but of two. Hence, since He is the mediator of God and men, it was fitting that He be God and man. For were he purely man or solely God, He would not be a true mediator. Therefore, if He is true God, then since no one is his own mediator, someone might suppose that there are, besides Him, other gods of whom He was the mediator. …

Commentary on Galatians ↗