6 Your boasting is not good.Do you not know that a little leaven leavens the whole lump of dough?
7 Clean out the old leaven so that you may be a new lump, just as you are in fact unleavened. For Christ our Passover also has been sacrificed.
8 Therefore let us celebrate the feast, not with old leaven, nor with the leaven of malice and wickedness, but with the unleavened bread of sincerity and truth.
1 Cor 5:8 has been suggested to have arisen from the controversy surrounding the date of the fast and passover celebration in the 2nd century catholic churches and came to a head under Pope Victor who threatened excommunication with anyone who had a problem with the Western practice of celebrating the passover on Easter.
This was the Quarto-deciman controversy. This situation continued until circa 190 CE. Eusebius records (Eccl. Hist., V, xxiii) that at the time of Pope Victor:
A question of no small importance arose at that time. The diocese of all Asia, as from an older tradition, held that the fourteenth day of the moon, on which day the Jews were commanded to sacrifice the lamb, should always be observed as the feast of the life-giving Pasch [epi tes tou sõterion Pascha eotes], contending that the fast ought to end on that day, whatever day of the week it might happen to be (Cath. Encyc., Vol. V, art. Easter, p. 228).The text itself seems to be recommending a settting aside the strictness of the passsover details yet expresssing distaste for the use of old pagan traditions (Easter). This seems to be the position of Ireneus who expressed his unhappiness with Victor's firm had yet himself preferred the Jewish date for the feast.