Jamieson, Fausset and Brown on Galatians 3:2

2. "Was it by the works of the law that ye received the Spirit (manifested by outward miracles, Ga 3:5; Mr 16:17; He 2:4; and by spiritual graces, Ga 3:14; Ga 4:5, 6; Ep 1:13), or by the hearing of faith?" The "only" implies, "I desire, omitting other arguments, to rest the question on this alone"; I who was your teacher, desire now to "learn" this one thing from you. The epithet "Holy" is not prefixed to "Spirit" because that epithet is a joyous one, whereas this Epistle is stern and reproving [Bengel].

hearing of faith--Faith consists not in working, but in receiving (Ro 10:16, 17).