A.T. Robertson Commentary Galatians 3:2

"This only would I learn from you. Received ye the Spirit by the works of the law, or by the hearing of faith?" — Galatians 3:2 (ASV)

This only (τουτο μονον). Paul strikes at the heart of the problem. He will show their error by the point that the gifts of the Spirit came by the hearing of faith, not by works of the law.