John Chrysostom

And thus this slight error, he says, if not corrected, will have power (as the leaven has with the lump) to lead you into complete Judaism.

Homily on Galatians 5 ↗

Theophylact of Ohrid

Lest they say: "why do you rebuke us so strongly (for we have violated only one commandment) and exaggerate our guilt?" – he says that this seemingly insignificant circumstance causes substantial harm. For, just as leaven, even a small amount, by itself leavens and transforms the entire lump of dough, so too circumcision, although it constitutes only one commandment, draws you into Judaism in all its fullness.

Commentary on Galatians ↗

Oecumenius

So that they might not say, "What great thing have we done by keeping only one commandment of the law?" he says, "One?" and what is that? "A little yeast infuses the whole batch of dough." For just as that infuses the whole batch, he says, so your circumcision will certainly bring in Judaism to perfection, unless you are corrected.

Commentary on Galatians ↗

Jerome

A little yeast leavens the whole batch of dough. It is incorrectly translated in our codices: A little yeast corrupts the whole lump, and the translator has conveyed the sense of the interpreter rather than the words of the Apostle. However, Paul himself uses this same statement to the Corinthians: where he commands that the one who had his father's wife be removed from their midst, and be handed over to destruction and affliction of the flesh through fasting and sickness, so that the spirit may be saved on the day of the …

Commentary on Galatians ↗

Thomas Aquinas

The second explanation is that they might suppose that consenting to a few is not a great matter, since it constitutes no danger. But he says that they must not consent to them at all, nor underestimate their artifices; rather they must oppose them at the start, because "a little leaven corrupteth the whole lump," i.e., those few who are persuading you. …

Commentary on Galatians ↗