In 1 Corinthians 2:7, Christ is called "Our Pasch," in the sense of "Our Paschal Lamb."
See also, lamb.
Exhorting to moral perfection, Saint Paul alludes to the ritual of the Jewish Passover.
After the sacrifice of the lamb all trace of yeast was removed from the house; and only unleavened bread was to be eaten during the week the festival lasted.
Christ is the true Paschal Lamb who taketh away the sins of the world; He has been offered up in the sacrifice of Calvary; we celebrate a paschal feast which is to last, not a week, but forever.
Christians must be free from the "leaven of wickedness," that is, from all moral corruption.
Leaven, or yeast, was the symbol of corruption and its rapid action was proverbial.
New Catholic Dictionary