Why does Paul repeat the same teaching in Galatians 3?
Paul repeats the same teaching because repetition is the mother of learning; hearers must hear these truths over and over before they soak in. In Galatians 3:3 he restates the Spirit-versus-flesh argument in different words, asking whether having begun in the Spirit they are now made perfect by the flesh, meaning the Law.
Les explains that verse 3 says the same thing as verse 1, only in a little different wording. Paul repeats it because repetition is the mother of learning, and until we hear these things over and over, they do not seem to soak in. He again asks, "Are you so foolish?" just as he said, "Oh foolish Galatians." Having begun in the Spirit—the Holy Spirit's work that generated salvation because of their faith—are they now made perfect, or right with God, by the flesh? The flesh refers to the Law, which was fleshly, weak, and beggarly. The Spirit was received by the hearing of faith, not by the works of the Law.