John Calvin Commentary Galatians 5:9

"A little leaven leaveneth the whole lump." — Galatians 5:9 (ASV)

A little leaven. This refers, I think, to doctrine, not to people. It guards them against the harmful consequences that arise from the corruption of doctrine, and warns them not to consider it, as is commonly done, to be a matter involving little or no danger. Satan’s strategy is that he does not attempt an open destruction of the whole gospel, but he taints its purity by introducing false and corrupt opinions. Many people 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 so that we do not allow any counterfeit to be substituted for the pure doctrine of the gospel.