Galatians 5:9

9.A little leaven. This refers, I think, to doctrine, not to men. It guards them against the mischievous consequences which arise from corruption of doctrine, and warns them not to consider it, as is commonly done, to be a matter attended by little or no danger. Satan’s stratagem is, that he does not attempt an avowed destruction of the whole gospel, but he taints its purity by introducing false and corrupt opinions. Many persons are thus led to overlook the seriousness of the injury done, and therefore make a less determined resistance. The apostle proclaims aloud that, after the truth of God has been corrupted, we are no longer safe. He employs the metaphor of leaven, which, however small in quantity, communicates its sourness to the whole mass. We must exercise the utmost caution lest we allow any counterfeit to be substituted for the pure doctrine of the gospel.