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.
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Just as a little leaven leavens the whole lump, an evil life corrupts the whole man. Therefore Paul wants us to avoid not only evil acts but all interest in sin, so that sincerity may cleanse our lives and truth may exclude all deception. Commentary on Paul’s Epistles.
Therefore let us keep the feast. The Latin has, "Let us banquet," because feasts were wont to be celebrated with solemn banquets in token of rejoicing.
The feast here is either the feast of the Passover or of unleavened bread. And notice that, according to Exod.xii, the evening of the fourteenth day of the month, or of the Passover, was not, strictly speaking, the feast, but the following morning was, which was called the feast of the first day of unleavened bread, and lasted for seven days, during which nothing but unleavened bread was allowed to be eaten; and before those days, viz, in the fourteenth day of the first month Nisan, instead of the Paschal lamb that had been killed, they killed other Paschal victims, viz, burnt offerings and peace-offerings. Cf. Numbers 28:19. The meaning, therefore, is this: Christ, having been sacrificed for us as our Passover, has redeemed us, and has begun for us the feast of unleavened bread. Therefore, after this Passover, after the death and red...
The old leaven refers among other things to the priests who allow an enormous amount of the old leaven to remain within the church, namely, the covetous, the extortioners and those guilty of doing anything that would keep them out of the kingdom of heaven.