Paul compares the Christian life to running (5:7). He usually compares it to walking (Col.1:10), i.e., walking in the same grace that saves us (Col.2:6). But the Galatians had learned to walk in grace so well that they’d broken into a run.
Then someone “hindered” them (Gal.5:7), a word that means stopped in that context (cf.IThes.2:18). Paul wanted to know “who,” because in so doing they stopped them from obeying the truth (Gal.5:7). The law used to be the truth, so God told the Jews to walk in it (Ex.16:14). But now grace is the truth, so walking in the law makes you disobedient to God’s truth.
Paul went on to point out that the God who called them (5:8 cf.ICor.1:9;IIThes.2:13,14) into grace (Gal.1:5,6) wasn’t the One who hindered them, so it must have been Satan. He’s an angel of light (IICor.11:14,15), and “the law is light” (Pr. 6:23). So Satan has his demonic ministers teach “leaven” (Gal.5:9), i.e., the doctrine (cf.Mt.16:6,11,12) of the law.
Leaven is like yeast though, and always spreads and leavens the whole lump of bread dough. Once Satan introduced the leaven of the law in Galatia, it spread throughout Christianity for the next 2,000 years. That’s why churches must oppose the law like they oppose sin. Sin will spread in an assembly too, so in speaking about the fornicator in Corinth, Paul also told the Corinthians that “a little leaven leaveneth the whole lump. But legalism must be just as dangerous, if Paul uses the exact same words to warn us about the law.
Paul doubted the Galatians would ever come out of legalism (Gal.4:20), but he had “confidence” in them “through the Lord” (Gal.5:10)—just as he had confidence in the Corinthians through Him, that they would come out of carnality (IICor.2:3;7:16;8:22). And since Paul’s words were actually God’s words, that means that however deeply you’ve fallen into sin or legalism, God is confident you’ll come out of it.
Paul must have had no confidence in the legalizer though, for instead he said he’d just have to “bear his judgment” (Gal.5:10). That shows Paul knew he wasn’t saved, for after the Jerusalem Council, saved Jews knew that men saved by grace without the law didn’t need to be put under the law.
Paul primarily meant that he’d have to bear his judgment someday in hell. But he may have been thinking of inflicting some judgment on him personally, the way he did with an unsaved man in Acts 13:10,11. Why else would he ask who the man was twice (Gal.3:1;5:7).
Next, the legalizer was saying that Paul preached circumcision and the law, and the Galatians believed it because he circumcised Timothy while in Galatia (Acts 16:1-3). But he didn’t do it because the Jews thought he couldn’t be saved without it. He did it because the Jews wouldn’t let him into their synagogues without it, and Paul chose Timothy to “go forth with him” into those synagogues to preach the gospel. That’s an example of I Corinthians 9:20-23.
But that’s a lot different than preaching circumcision and the law. Paul dispels that rumor by pointing out that if he preached that, the Jews would stop persecuting him because his message would cease to offend them. It offends people even today when you tell them you can be saved without the law!
Earlier Paul made it sound like there was only one legalizer (Gal.5:10), but the word “they” (v.12) shows there were more. It had spread like leaven! He wanted them “cut off,” a play on words. If you weren’t circumcised in time past, you were “cut off” from God’s people (Gen.17:14). Paul is saying he wanted those legalizers cut off out of the assembly, like the leaven of the fornicator in I Corinthians 5:2,13.
I once taught that Paul wished them castrated, making a play on words on circumcision. But castrating a legalizer would not stop legalism from spreading, and that’s the context.