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.

A little leaven - This is evidently a proverbial expression; see it explained in the notes at 1-Corinthians 5:6. Its meaning here is, that the embracing of the errors which they had adopted was to be traced to some influence existing among themselves, and acting like leaven. It may either mean that there was existing among them from the first a slight tendency to conform to rites and customs, and that this had now like leaven pervaded the mass; or it may mean that the false teachers there might be compared to leaven, whose doctrines, though they were few in number, had pervaded the mass of Christians; or it may mean, as many have supposed, that any conformity to the Jewish law was like leaven. If they practiced circumcision, it would not stop there. The tendency to conform to Jewish rites would spread from that until it would infect all the doctrines of religion, and they would fall into the observance of all the rites of the Jewish law. It seems to me that the second interpretation referred to above is the correct one; and that the apostle means to say, that the influence which had brought this change about was at first small and unimportant; that there might have been but a few teachers of that kind, and it might have not been deemed worthy of particular attention or alarm; but that the doctrines thus infused into the churches, had spread like leaven, until the whole mass had become affected.

A little leaven leaveneth the whole lump - A proverbial expression, see 1-Corinthians 5:6, very aptly applied to those who receive the smallest tincture of false doctrine, relative to the things essential to salvation, which soon influences the whole conduct, so that the man becomes totally perverted. They might have argued, "It is a small thing, and should not be made a subject of serious controversy, whether we be circumcised or not." Granted, that in itself it is a small matter; but, as every man who is circumcised is a debtor to do the whole law, Galatians 5:3, then your circumcision leads necessarily to your total perversion; as the little portion of leaven, mixed with the batch, soon leavens the whole lump.

(8) A little leaven leaveneth the whole lump.
(8) He adds this, that he may not seem to contend upon a trifle, warning them diligently (by a metaphor which he borrows of leaven, as Christ himself also did) not to allow the purity of the apostolic doctrine to be infected with the least corruption at all.

A little leaven leaveneth the whole lump. A proverbial expression pretty much in use with the Jews; see 1-Corinthians 5:6, respecting either persons or things; and is in answer to, or prevents an objection that might be made, or something that might be said, in favour of these churches; that their case was not so bad, or the danger they were in so great, as was represented by the apostle; since they were only a few persons that propagated this notion, and but few that received it, at least thoroughly gave into it; and that, if it was an error, it was but a small one, and only regarded a single ritual, or a few rituals of the law; to which the apostle replies, by supposing, but not granting this to be the case, since they were pretty generally declined, and the error was not a slight one, that as a little sour leaven influences and ferments a large mass, or lump of dough, and makes it of the same nature with it, so a small error in doctrine, as it may be thought to be, increases to more ungodliness, and eats as doth a canker; and though a few hands may be first concerned in propagating it, and but few be infected with it, yet these may soon spread the contagion through the whole society: wherefore errors and false teachers should be nipped in the bud, and stopped in their beginnings, how inconsiderable soever they and their tenets may be judged to be.

A little leaven--the false teaching of the Judaizers. A small portion of legalism, if it be mixed with the Gospel, corrupts its purity. To add legal ordinances and works in the least degree to justification by faith, is to undermine "the whole." So "leaven" is used of false doctrine (Matthew 16:12 : compare Matthew 13:33). In 1-Corinthians 5:6 it means the corrupting influence of one bad person; so BENGEL understands it here to refer to the person (Galatians 5:7-8, Galatians 5:10) who misled them. Ecclesiastes 9:18, "One sinner destroyeth much good" (1-Corinthians 15:33). I prefer to refer it to false doctrine, answering to "persuasion" (Galatians 5:8).

A little leaven leaveneth the whole lump - One troubler, Galatians 5:10, troubles all.