The leaven proverb (5:9)

Says “A little leaven works through the whole batch of dough” (Gal 5:9; μικρὰ ζύμη ὅλον τὸ φύραμα ζυμοῖ). It stands between “Who has obstructed you from obeying the truth?” (5:7) and “The one who is troubling you will bear the judgment, whoever he may be” (5:10). Paul uses the same words elsewhere: “Do you not know that a little leaven works through the whole batch of dough?” (1 Cor 5:6).

Implies Because the saying comes between the question of who obstructed them (5:7) and the judgment on the one troubling them (5:10), and Paul repeats it word for word in 1 Cor 5:6, the text uses a known saying to warn that a small influence spreads through the whole.

Uncertain Whether the “little leaven” of 5:9 is the teaching, the teacher, or both, is open; no source logged for this chapter comments on the saying, and this background does not fix the meaning of the text.