What does Galatians 3:3 mean?

This letter is written to the people in the region of Galatia who believed in Christ when Paul came to them with the gospel. Unfortunately, they have begun to think they need to live like the Jews in order to be truly saved (Galatians 2:4). Paul has called them "foolish," with loving purpose (Galatians 3:1), and now asks if they are, in fact, foolish.

In the previous verse, Paul asked them to remember when they received the Holy Spirit. Specifically, he asks if the Spirit came to them after they believed, or after they did some works of the law. The clear answer is that the Spirit came to live inside them, and manifested in some obvious supernatural ways when they believed. This often happened in the early church, when groups believed in large numbers.

Paul's point is fairly clear: if the Spirit came when they believed, why would they think they needed to start following the law to be acceptable to God?

Now, Paul asks that if they began in the power of God's Spirit, do they think they will be perfected by their own human efforts—"the flesh"—to live according to the law? Again, the implied answer is "obviously not."