Thematic Bible: A symbol for sin

Your boasting is not good. Do ye not know that a little leaven leavens the whole lump? Purge out the old leaven, that ye may be a new lump, according as ye are unleavened. For also our passover, Christ, has been sacrificed; so that let us celebrate the feast, not with old leaven, nor with leaven of malice and wickedness, but with unleavened bread of sincerity and truth.