Before Passover, to this day, Jewish homes are cleaned of every crumb of leaven (chametz): the Mishnah opens its Passover tractate with the search for leaven by candlelight on the night before the feast (Pesachim 1:1), and the search is still done, traditionally by candle, with the last crumbs burned in the morning. Paul's readers knew that yearly house-cleaning; his argument leans on it: the lamb is already sacrificed (“Christ, our Passover lamb”), so the house, the congregation, must be cleansed of the old leaven to match. The feast supplies both halves of the picture: the lamb and the purge.