Bible Dictionary
Leaven
Leaven was fermented dough kept from an earlier baking and mixed into new dough. It was excluded from most offerings and from Passover, and Jesus and Paul used it as a picture of both pervasive influence and corrupting doctrine or sin.
Biblical Usage
Leaven was excluded from most offerings and from Passover. Jesus used it both for its hidden, spreading action in illustrating the growth of the kingdom of heaven and for corrupt doctrine. Paul used it as a figure of sin and moral corruption.
Scripture References