Paul asks another rhetorical question to remind the Galatians how people receive the Spirit. Alternate translation: "He ... does not do it by the works of the law; he does it by hearing with faith." (See: [[rc://en/ta/man/translate/figs-rquestion]])
Your language may require that what the people heard and whom they trusted be stated explicitly. Alternate translation: "because you heard the message and had faith in Jesus" or "because you listened to the message and trusted in Jesus" (See: [[rc://en/ta/man/translate/figs-explicit]])