Note 17 at Gal. 5:9: Paul uses leaven (yeast) twice in his New Testament writings (1 Cor. 5:6; Gal. 5:9). Leaven symbolizes evil or sin in scripture (see note 1 at Mt. 13:33, p. 200). Even the smallest influence of a false gospel will eventually corrupt all, causing the whole system of grace to fall.