Why did Paul call the Galatians foolish?

In Galatians 3:1, Paul expresses frustration at the Galatians' decision to abandon the truth of the gospel in favor of the law and works. He views this shift as a sign of unthinking behavior and lack of discernment—hence calling them foolish. The term denotes those who are thoughtless, failing to weigh the implications of leaving Christ's grace for a system that can never justify. His rhetorical questions highlight their foolishness in thinking they could achieve perfection through the flesh after beginning in the Spirit.