What is leaven and how does yeast work in dough?
Leaven is yeast. When yeast is placed into dough, it does not affect only a portion; it permeates and leavens the entire lump. Scripture uses this as a picture: a little leaven leavens the whole lump, and false teaching likewise spreads until it permeates everything.
Les explains that in common vernacular leaven is yeast. When yeast is put into dough, it is not going to leaven just a fourth of it—it will leaven all of it. He applies this to false teaching: if you let a little false teaching in as leaven, it is not long until it permeates the whole, and one thing leads to another. He points out that Christendom in general has been leavened, and Christianity today is leavened. Things Scripture and Christianity stood against 50, 60, or 70 years ago are now part and parcel of everyday experience. He illustrates with a church they visited when one of their children was at the university: the world came knocking, the church said it could accommodate that, then the door opened further and more worldliness came in, and the church said it could live with that. Now the doors are wide open, and you cannot tell the difference between the church and the world. A little leaven leavened the whole lump. Yet Paul did not give up on these people; he still had confidence in them and trusted that God would bring them back to the truth.