5:7-12 The life of a Christian is a race, wherein he must run, and hold on, if he would obtain the prize. It is not enough that we profess Christianity, but we must run well, by living up to that profession. Many who set out fairly in religion, are hindered in their progress, or turn out of the way. It concerns those who begin to turn out of the way, or to tire in it, seriously to inquire what hinders them. The opinion or persuasion, ver. 8, was, no doubt, that of mixing the works of the law with faith in Christ in justification. The apostle leaves them to judge whence it must arise, but sufficiently shows that it could be owing to none but Satan. It is dangerous for Christian churches to encourage those who follow, but especially who spread, destructive errors. And in reproving sin and error, we should always distinguish between the leaders and the led. The Jews were offended, because Christ was preached as the only salvation for sinners. If Paul and others would have admitted that the observance of the law of Moses was to be joined with faith in Christ, as necessary to salvation, then believers might have avoided many of the sufferings they underwent. The first beginnings of such leaven should be opposed. And assuredly those who persist in disturbing the church of Christ must bear their judgment.
9. A little leaven—the false teaching of the Judaizers. A small portion of legalism, if it be mixed with the Gospel, corrupts its purity. To add legal ordinances and works in the least degree to justification by faith, is to undermine "the whole." So "leaven" is used of false doctrine (Mt 16:12: compare Mt 13:33). In 1Co 5:6 it means the corrupting influence of one bad person; so Bengel understands it here to refer to the person (Ga 5:7, 8, 10) who misled them. Ec 9:18, "One sinner destroyeth much good" (1Co 15:33). I prefer to refer it to false doctrine, answering to "persuasion" (Ga 5:8).