Paul wants the Galatians to settle the debate from their own lives. They had received the Spirit. How did it happen? Was it by keeping the law, or by hearing the gospel and believing it?

"This only would I learn of you" shows that one question is enough. Gill says it is "this one question, which, if rightly attended to, and honestly answered, must expose their folly, and put an end to the controversy." Barnes notes Paul is "retaining still the language of severe reproof." Matthew Henry says Paul appeals to the Spirit they had received when they became Christians, noting that many of them at least had also received miraculous gifts (Galatians 3:5).

Paul does not record their answer because it is obvious. They received the Spirit by the hearing of faith. Matthew Henry puts it simply: "It was not by the first, but the last." Paul repeats the question in Galatians 3:5 and then points to Abraham, who "believed God, and it was accounted to him for righteousness" (Galatians 3:6). He closes the argument with the purpose of Christ's work: "that we might receive the promise of the Spirit through faith" (Galatians 3:14).

Many of the Galatians had once done "service unto them which by nature are no gods" (Galatians 4:8). Now some teachers were pressing them to be circumcised and keep the law (Galatians 5:2; 6:12).

"The hearing of faith" means hearing the gospel with faith. Gill explains it as "the preaching of it, the report of it," through which the Spirit is received. Barnes describes it as hearing the gospel "requiring faith as a condition of salvation." Paul sets this against "the works of the law," doing what the law commands as the way to be accepted.

This question shows how God gives His Spirit: as a gift received by faith, not a wage earned by law-keeping. The Galatians' own story proved it. They had received the Spirit through the gospel Paul preached, not by keeping the law (Galatians 3:1-2).

Paul builds the rest of the chapter on this point. Abraham was counted righteous by believing (Galatians 3:6), and Christ redeemed us from the curse of the law "that we might receive the promise of the Spirit through faith" (Galatians 3:13-14). What God starts by grace through faith, He does not hand over to human effort (Galatians 3:3).