Galatians 3:6

The apostle having reproved the Galatians for not obeying the truth, and endeavoured to impress them with a sense of their folly herein, in these verses he largely proves the doctrine which he had reproved them for rejecting, namely, that of justification by faith without the works of the law. This he does several ways. I. From the example of Abraham's justification. This argument the apostle uses, Rom. 4. Abraham believed God, and that was accounted to him for righteousness (Gal 3:6); that is, his faith fastened upon the word and promise of God, and upon his believing he was owned and accepted of God as a righteous man: as on this account he is represented as the father of the faithful, so the apostle would have us to know that those who are of faith are the children of Abraham (Gal 3:7), not according to the flesh, but according to the promise; and, consequently, that they are justified in the same way that he was. Abraham was justified by faith, and so are they. To confirm this, the apostle acquaints us that the promise made to Abraham (Gen 12:3), In thee shall all nations be blessed, had a reference hereunto, Gal 3:8.

— Matthew Henry's Concise Commentary (public domain)

The answer to the question in Gal 3:5 is here taken for granted, It was by the hearing of faith: following this up, he says, "Even as Abraham believed," &c. (Gen 15:4-6; Rom 4:3). God supplies unto you the Spirit as the result of faith, not works, just as Abraham obtained justification by faith, not by works (Gal 3:6, Gal 3:8, Gal 3:16; Gal 4:22, Gal 4:26, Gal 4:28). Where justification is, there the Spirit is, so that if the former comes by faith, the latter must also.

— Jamieson-Fausset-Brown Commentary (public domain)