Galatians 5:9

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 …

(Verse 9.) I trust in you in the Lord, that you will understand nothing else. Not by conjecture, as some would have it, but by the prophetic spirit, Paul declares that the Galatians will return to the way of truth they had lost (I Cor. XII). For indeed, he who encouraged others to emulate the charisms, especially prophecy, himself spoke with the same abundant grace: For we know in part, and we prophesy in part (Ibid., XIII, 9).

For an offender, when he sees any other doing as bad as himself, will be encouraged to do the very same things; and then the wicked one, taking occasion from a single instance, works in others, which God forbid: and by that means the flock will be destroyed. For the greater number of offenders there are, the greater is the mischief that is done by them: for sin which passes without correction grows worse and worse, and spreads to others; since "a little leaven infects the whole lump," and one thief spreads the abomination over a whole nation and "dead flies spoil the whole pot of sweet ointment;" and "when a king hearkens to unrighteous counsel, all the servants under him are wicked." So one scabbed sheep, if not separated from those that are whole, infects the rest with the same distemper; and a man infected with the plague is to be avoided by all men; and a mad dog is dangerous to every one that he touches. If, therefore, we neglect to separate the transgressor from the Church of God, we shall make the "Lord's house a den of thieves."

(Verse 9.) 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 Lord Jesus Christ (some manuscripts add: our Lord). He says indeed: Not good is your boasting. Do you not know that a little leaven leavens the whole lump? (I Cor. 5:5, 6, and following)? Or (as we have now corrected) leavens the whole mixture? And immediately he adds: Purge out the old leaven, that you may be a new mixture, as you are unleavened. For Christ our Passover is sacrificed for us. Therefore let us keep the feast, not with old leaven, neither with the leaven of malice and wickedness, but with the unleavened bread of sincerity and truth. But now, according to this same sentiment, He teaches that the spiritual bread of the Church, which came down from heaven, should not be violated by Jewish interpretation; and the Lord Himself commanded the same to His disciples, that they should beware of the leaven of the Pharisees (John VI). And the evangelist, making this more clear, added: But He had spoken to them concerning the doctrine of the Pharisees (Matthew XVI). Moreover, what is this other doctrine of the Pharisees, if not the observance of the Law according to the flesh? Therefore, this is the meaning: Do not think lightly of men who come from Judea and teach another doctrine, for they despise danger. […]

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.