Галатам 5:9

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 (Mat 16:12 : compare Mat 13:33). In Co1 5:6 it means the corrupting influence of one bad person; so BENGEL understands it here to refer to the person (Gal 5:7-8, Gal 5:10) who misled them. Ecc 9:18, "One sinner destroyeth much good" (Co1 15:33). I prefer to refer it to false doctrine, answering to "persuasion" (Gal 5:8).

In this chapter the apostle comes to make application of his foregoing discourse. He begins it with a general caution, or exhortation (Gal 5:1), which he afterwards enforces by several considerations (Gal 5:2-12). He then presses them to serious practical godliness, which would be the best antidote against the snares of their false teachers; particularly, I. That they should not strive with one another (Gal 5:13-15). II. That they would strive against sin, where he shows, 1. That there is in every one a struggle between flesh and spirit (Gal 5:17). 2. That it is our duty and interest, in this struggle, to side with the better part (Gal 5:16, Gal 5:18). 3. He specifies the works of the flesh, which must be watched against and mortified, and the fruits of the Spirit, which must be brought forth and cherished, and shows of what importance it is that they be so (Gal 5:19-24). And then concludes the chapter with a caution against pride and envy.

5:9 This verse was apparently a well-known adage (literally A little yeast makes the whole loaf rise; cp. Matt 13:33; 16:6, 11-12; 1 Cor 5:6-7). A little reliance on the law for acceptance by God soon results in forgetting that Christ saves by his work alone.