A little leaven leaveneth the whole lump. A little leaven communicates its bitterness to the whole mass of meal. This is a maxim describing the way that a vicious part spoils the whole, and of course is capable of general application. In 1 Corinthians 5:6 it is applied to the fornicator who was corrupting the whole Corinthian Church, and here it is applied to the Judaisers, who are being dealt with throughout this chapter, and declares that they are corrupting the whole of the Galatian Church. Jerome says: "Arius in Alexandria was but a single spark, but not being at once extinguished, he grew to a flame, and devastated the whole world. For their word eateth the body as a canker, and the rot in a single sheep infects the whole flock."
The maxim may be yet more fitly applied to the doctrine itself of the Judaisers, in the sense that a single error in the faith, such as that about the necessity of the law, overturns the whole faith. Chrysostom and Theophylact apply it, yet more particu...
All leaven corrupts the bread, and the corrupted bread is flour. When the mass of flour is left, it sours, and then comes the leavening. Now when a small amount of the leaven is put into the mass, the mass is corrupted. “You,” he says, “must be unleavened bread. Therefore that little addition of yours, which you thought a small amount, namely, your observing of circumcision and the rest, because it is corrupt, corrupts the mass of o ur gospel. If so, you do not have full hope in Christ, and neither does Christ regard you as his own or people whose hope depends on him. For it is faith that sets free, and, as I have said, he has no faith who hopes for any sort of help apart from Christ, even along with Christ.” .
Some may say, “Why do you blow up the matter so portentously by your words? We have kept a single command of the law, and are you clamoring because of that?” But Paul is concerned not only for their present but for their future…. “For,” he says, “this little evil, if uncorrected, has the strength to lead you into complete Judaizing, just as leaven acts on the lump.”
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"
