What does Galatians 5:9 mean?

Would Paul's readers think he was making too big of a deal out of the issue of circumcision? Earlier, the Galatians had accepted Paul's teachings (Galatians 5:7). They'd accepted that salvation was by faith alone, before being distracted by a claim that rituals and circumcision were also required (Galatians 2:4). Paul has asked the Galatians who was leading them astray. Paul was making it personal now, suggesting, in essence, that they name names. This man was a false teacher, and his teaching should not be allowed to stand among them.

Paul now shares a familiar proverb meant to illustrate his point, and to show that his concern is not overblown. Leaven—or yeast—does not remain isolated in a single spot within a lump of dough. It spreads and becomes part of the entire loaf. In a similar way, false teaching and its consequences are never limited to just a few people in a group. It is either rejected—and removed—or it works its way into the thoughts and practices of the whole group. False ideas are contagious; they spread and take hold.

Paul desperately wants the Galatian believers to identify the false teaching about adding works to faith in Christ, so that they can remove it from among them.