“Therefore 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.”
King James Version (KJV)
5:8 Therefore let us keep the feast - Let us feed on him by faith. Here is a plain allusion to the Lord's supper, which was instituted in the room of the passover. Not with the old leaven - Of heathenism or Judaism. Malignity is stubbornness in evil. Sincerity and truth seem to be put here for the whole of true, inward religion.
1Co 5:8 Let us keep the feast. "Let us keep feast", or "festival". There is no article in the Greek. The reference is not to the Lord's Supper, or to Easter, as some have supposed, so much as to a constant duty. We always have a Paschal Lamb; hence it is always our duty to keep festival by casting out all leaven; either the old leaven of heathen vice, or of the leaven of malice and wickedness, or any sin.