'Leaven' in the Bible

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

Purge out the old leaven, that ye may be a new lump, even as ye are unleavened. For our passover also hath been sacrificed, even Christ:

wherefore 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.