What does yeast do to a batch of bread dough?
Yeast, also called leaven, affects the entire batch of bread dough once introduced; it permeates the whole lump. Scripture uses leaven to picture evil or sin, which similarly spreads through a whole body unless it is rooted out and removed.
Les explains that leaven in Scripture always denotes evil or sin, and leaven is yeast. When yeast is put into bread dough, it will affect the whole lump. That is the teaching of Scripture: whenever leaven comes in—evil—unless it is rooted out, it will sooner or later affect the whole. In the Exodus account, the Israelites were to leave leaven out of their bread dough and bake unleavened cakes. This pictured them as a separated people, separated unto God and no longer wrapped up in the paganism of their Egyptian masters.