This rhetorical question shows that Paul is surprised and even angry that the Galatians are foolish. Alternate translation: "You are very foolish!" (See: [[rc://en/ta/man/jit/figs-rquestion]])
The word "flesh" is a metonym for effort. Alternate translation: "by your own effort" or "by your own work" (See: [[rc://en/ta/man/jit/figs-metonymy]])