This expression is found twice in the scriptures.  Let’s first look at 1 Corinthians 5:1-6, “It is reported commonly that there is fornication among you, and such fornication as is not so much as named among the Gentiles, that one should have his father’s wife. And ye are puffed up, and have not rather mourned, that he that hath done this deed might be taken away from among you. For I verily, as absent in body, but present in spirit, have judged already, as though I were present, concerning him that hath so done this deed, In the name of our Lord Jesus Christ, when ye are gathered together, and my spirit, with the power of our Lord Jesus Christ, To deliver such an one unto Satan for the destruction of the flesh, that the spirit may be saved in the day of the Lord Jesus. Your glorying is not good. Know ye not that a little leaven leaveneth the whole lump?”

What a shameful sin was being committed in the Corinthian church.  One man was having a sexual relationship with his father’s wife…his stepmother.  So appalling was this sin, that the Apostle noted that not even the Gentiles (the unsaved) committed such as act.  What did the local church do about this man who was living in such sin?  It seems they did nothing about it.  Rather, they became puffed up, or boastful, that they were so tolerant of such a great sin.  If nothing else, at least they could have mourned over this wicked deed. Paul’s judgment was that this man needed to be “taken away from you” or excommunicated from this local church until he repented of his deeds.  He instructed the church to administer this punishment upon this sinning member of the church. 

By telling these people that “a little leaven leaveneth the whole lump” Paul was telling them that by acknowledging MORAL SIN in their midst but doing nothing about it, they became just as guilty as the one who was living in sin.  Even as yeast (leaven) in a loaf of bread will cause the whole loaf to rise, so unjudged sin in the assembly will affect the whole group. 

Now let’s read Galatians 5:1-4, “Stand fast therefore in the liberty wherewith Christ hath made us free, and be not entangled again with the yoke of bondage. Behold, I Paul say unto you, that if ye be circumcised, Christ shall profit you nothing. For I testify again to every man that is circumcised, that he is a debtor to do the whole law. Christ is become of no effect unto you, whosoever of you are justified by the law; ye are fallen from grace.”  In the Galatian church, there were those who began to teach that a person must be circumcised before he can truly be saved.  The Lord tells us through Paul that if we are circumcised as a requirement for salvation, we set aside the finished work of Christ on the cross.  To say that something other than faith is needed to be saved is a terrible and dishonoring doctrine.  We go on to read in verse 9, “A little leaven leaveneth the whole lump.” 

We learn here that DOCTRINAL SIN that is tolerated in the church is also like yeast which causes the whole loaf to rise.  For the church to know the truth but allow someone to teach something that is clearly contrary to the scriptures, the Lord will hold them just as responsible as the one who teaches the false doctrine.  To mix law and grace is a terrible destructive teaching that will strangle the spiritual growth of the believer and cause dishonor to the name of Christ.  To honor the Lord, we must get rid of the false doctrine that is being taught.  We can do that by confronting the teacher of false doctrine with the proper truth as taught in the Bible.  If he accepts the correction and ceases to teach sinful doctrine, then the leaven is gone from the assembly.  If the teacher refuses to accept the correction and continues to teach false doctrine, then they would face excommunication even as the man caught up in moral sin.  (Read Matthew 18:15-17 to read the proper procedure before excommunicating a fellow believer for moral or doctrinal sin.)  Of course, any time excommunication becomes necessary, the restoration of that disciplined believer should be sought.  (484.6)