How does a little false teaching affect a church or a person?

A little false teaching acts like leaven or yeast: it does not stay contained but permeates the whole lump, spreading until it influences the entire church or person. One error leads to another, and eventually the church can become leavened, indistinguishable from the world.

Les explains that leaven is yeast; when it is put into dough, it does not leaven only a fourth of it but leavens all of it. It is the same with false teaching: if a little bit of false teaching is let in as leaven, it is not long until it permeates the whole, and one error leads to another. He points to Christendom in general as having been leavened, saying Christianity today is leavened, and that things Scripture and Christianity stood against 50, 60, or 70 years ago are now part of everyday experience. He illustrates a church that first accommodated worldly things, then opened the door further, until you could not tell the difference between the church and the world. Paul, however, did not give up on these people but trusted that they would be brought back to the truth.

Galatians 5:9

A little leaven leaveneth the whole lump.

What happens when a little false teaching is allowed into a church?

A little false teaching acts like leaven: it does not stay contained. Once admitted, it spreads through the whole church, one error leading to another, until the entire body is permeated. Over time, the church accommodates worldly things and becomes indistinguishable from the world. This is what happens when a little leaven leavens the whole lump.