'Leaven' in the Bible

Your glorying is not good. Know ye not that a little leaven leaveneth the whole lump?

Purge out therefore the old leaven, that ye may be a new lump, as ye are unleavened. For even Christ our passover is sacrificed for us:

Therefore let us keep the feast, not with old leaven, neither with the leaven of malice and wickedness; but with the unleavened bread of sincerity and truth.