What does it mean that a little leaven leavens the whole lump (Galatians 5:9)?

(1) In the first place, Paul reminds them of what is common knowledge, even apart from the scriptures: “Know ye not that a little leaven leavens the whole lump?” The nature of leaven is such that when a small particle of it is added to a mass of pure dough, its special character is invariably imparted to the whole lump. This diffusive action of leaven is automatic, and requires no human aid. Silently and imperceptibly, the subtle influence is exercised until the secret process of pervasion and amalgamation is completed, and the final result is visible. Hence the baker, after kneading his dough, waits until it is leavened before making up his oven fire for the baking (Hosea 7:4, R.V. ). There is no need that he should assist the leaven in its work of leavening the dough, though its action is more rapid and effective when screened from the light.

A little leaven very soon leavens the whole lump (v.9). Leaven (yeast) always speaks of the corrupting action of sin. Being only a little, it may hardly be noticed at first, but deceitful men know how to gradually introduce their false doctrines and soon corrupt the truth. Only a little of the doctrine of self-righteousness added to Christianity will spoil the whole, for it magnifies man and belittles Christ. But Christianity is Christ highly exalted and mankind humbled to the dust. This same quotation as to leaven occurs also in 1 Corinthians 5:6 where self-indulgence (moral corruption) is the leaven, while in Galatians self-confidence (doctrinal evil) is the leaven. In either case it is mixture hated by God.

The Galatians however were turning aside not only from obedience to the truth, but from the truth itself. They had been persuaded to embrace these new ideas, which did not come from the God who had called them; and further, they had to remember that ideas and doctrines can work like leaven. They might be flattering themselves that they had only embraced a few minor items of Judaism, yet thereby they might become wholly Judaized.

It is written then that "a little leaven leavens the whole lump." Hence to many, as the same words appear in two different passages, the too rapid inference is that they point to the same thing. So far is this from being true that the application is wholly different. What then is the bearing of each? Pay attention to the principle, that, if we wish to understand any verse of scripture we must always interpret it by its context. In 1 Cor. 5 leaven represents what is unclean, corrupting, and manifestly immoral. They were not to allow "the wicked person" in their midst, for evil spreads; and ever so little leaven, if allowed, defiles the whole lump. In Galatia evil was taking what we may call a religious or legal form (Gal. 5:9). The Christians there were observing days, months, times, and years. They were crying up circumcision as a desirable supplement to faith. This was the Pharisaic leaven, as in Corinth was the Sadducean. The leaven of the Sadducees was the evil of free thought and licentious action. The leaven of the Pharisees was that of rigorous legalism and human tradition, as that of Herod was worldly time-serving.

"Ye did run well. Who did hinder you, that ye should not obey the truth? This persuasion cometh not of him that calleth you." He reproaches them with having listened to these false teachers, who had pressed circumcision. "A little leaven leaveneth the whole lump." Is it not solemn to find that the very word "leaven" which is used in 1 Corinthians to describe frightful moral corruption, in Galatians characterizes the introduction of the legal system among the children of God? God treats it as a most offensive thing. Indeed, the tone of the Holy Ghost in writing to the Galatians is even more severe than in addressing the Corinthians. Because, although the Corinthians were guilty of what was far more blameable in the sight of men, the Galatians had fallen into an error that struck more deeply at the foundations of God's grace; and a spiritual man invariably judges sin, not by that which man thinks of it, but by what it is in the sight of God. Having brought out the character of it, he says, "I have confidence in you through the Lord that ye will be none otherwise minded." He could not say that about all of them: he says it in a general way; and adds, "But he that troubleth you shall bear his judgment, whosoever he be." He wants to separate them and give a sense of horror about those who had misled them. "Faith which worketh by love" does not hesitate to use strong language about the corrupters of the Church of God — denounces them most earnestly, and as a duty to God and man. "I would they were even cut off which trouble you." He that troubleth you shall bear his judgment, whosoever he be." There were several engaged in that bad work. "And I, brethren, if I preach circumcision, why do I yet suffer persecution?" They had made the Apostle Paul to be a sort of evidence in their favour. They may have taken advantage of his circumcising Timothy, in order to make a show of inconsistency between his acts and his preaching. But St. Paul was not acting contrary to these principles when he circumcised Timothy. It was the elasticity of a man who could stop the mouths of objectors; and Paul, to silence Jewish slander, ended that question most unjewishly — by having Timothy circumcised. But he would not suffer it in the case of Titus, (who was a Greek,) whom he took up to Jerusalem with himself. This might appear capricious, but grace knows the time to be firm as well as to bend. There seems here to be an allusion to this, in his argument with the defenders of the law. It requires the wisdom of the Spirit of God giving one to know where one may use our liberty, or where it is a duty to stand as firm as a rock; and Paul did both. If Timothy had been circumcised, it was grace stopping mere fleshly questions, and not law, for his father was a Greek. But as to preaching it, such a thing was far from his mind. Had he ever pressed circumcision, he would have had their favour and countenance in every place that he visited. On the contrary, he was persecuted because he would not allow the flesh nor the title of circumcision.

We have already noticed the words in I Corinthians 5: 6, 7, "Know ye not that a little leaven leaveneth the whole lump? Purge out therefore the old leaven, that ye may be a new lump." There we find moral evil spoken of as leaven that needs to be purged out lest it leaven the whole lump-the entire Assembly. We also find the same words, "A little leaven leaveneth the whole lump" in Galatians 5:9. In studying this Epistle of Galatians we find that the leaven which the apostle referred to, and that by which the Galatian Assemblies were in danger of being leavened, was evil teaching by some as to the Gospel. The Gospel was perverted by these teachers and thus the fundamentals of the Christian faith were attacked.

Verse 9. "A little leaven leaveneth the whole lump." In this proverbial sentence, the weighty import of which is attested by its occurrence in two separate epistles, we are warned of the danger of entertaining in any form or measure, either doctrine or practice that is foreign to the simplicity of the Gospel… In the language of the New Testament, leaven always means something of the old and condemned nature, as opposed to the new. It is capable therefore, of a wide and varied application, and we accordingly find it so treated in the passages where it occurs. Moral iniquity and doctrinal perversion are its two chief meanings, as the occasions on which this proverb is repeated. clearly prove (cf. 1 Cor. 5:6). The Corinthian believers were endangered and defiled through their indifference to the gross moral evil which was suffered to go without condemnation or rebuke in the midst of their assembly. The Galatian churches, on the other hand, were contracting a defilement of a still more fatal kind, by consenting to doctrines which perverted, and thus destroyed, the Gospel of the grace of God. In both cases, the majesty of holiness and truth was outraged by the same means; by the admission, namely, of the old leaven under different forms. Things incongruous with Christ, and contrary to the mind of the 'Holy Spirit of God, were tolerated and accepted. Holiness was sacrificed in the one instance to a false view of Christian liberty; and in the other, truth was corrupted by human additions to the testimony of the Holy Ghost.

Paul writes to the Corinthians, with regard to evil practice, "know ye not that a little leaven leaveneth the whole lump?" (1 Cor. 5:6.) And to the Galatians, with regard to evil doctrine, subversive of Christianity, "A little leaven leaveneth the whole lump" (Gal. 5:9).

There the "leaven" is moral evil, as in the Gospels it was doctrinal evil. In Gal. 5:9 (the only remaining passage), it is again doctrinal. "Christ is become of no effect unto you whosoever of you are justified by the law … Ye did run well; who did hinder you that ye should not obey the truth? This persuasion cometh not from Him that calleth you. A little leaven leaveneth the whole lump."

Leaven is a type of evil. "Beware", said the Lord afterwards to His disciples, "of the leaven of the Pharisees." Their teaching was corrupt and corrupting. Nothing spreads so rapidly amongst the children of God as leaven. If it is introduced into a company, it spreads throughout that company: "a little leaven leavens the whole lump", the apostle said to the Corinthians (1 Cor. 5:6), and also to the Galatians (Gal. 5:9). He did not only mean that it will go on to leaven it, if not cast out. But the sentence also implies that the effect of the presence of leaven is to contaminate the whole lump, and that result is owing to the corrupting nature of the leaven itself. Unchecked evil has an insidious, permeating effect, destructive of what is good.