1 Corinthians 5:7

Here the apostle exhorts them to purity, by purging out the old leaven. In this observe, I. The advice itself, addressed either, 1. To the church in general; and so purging out the old leaven, that they might be a new lump, refers to the putting away from themselves that wicked person, Co1 5:13. Note, Christian churches should be pure and holy, and not bear such corrupt and scandalous members. They are to be unleavened, and should endure no such heterogeneous mixture to sour and corrupt them. Or, 2. To each particular member of the church. And so it implies that they should purge themselves from all impurity of heart and life, especially from this kind of wickedness, to which the Corinthians were addicted to a proverb. See the argument at the beginning. This old leaven was in a particular manner to be purged out, that they might become a new lump. Note, Christians should be careful to keep themselves clean, as well as purge polluted members out of their society. And they should especially avoid the sins to which they themselves were once most addicted, and the reigning vices of the places and the people where they live. They were also to purge themselves from malice and wickedness - all ill-will and mischievous subtlety. This is leaven that sours the mind to a great degree.

— Matthew Henry's Concise Commentary (public domain)

old leaven--The remnant of the "old" (Eph 4:22-24) heathenish and natural corruption. The image is taken from the extreme care of the Jews in searching every corner of their houses, and "purging out" every particle of leaven from the time of killing the lamb before the Passover (Deu 16:3-4). So Christians are continually to search and purify their hearts (Psa 139:23-24). as ye are unleavened--normally, and as far as your Christian calling is concerned: free from the leaven of sin and death (Co1 6:11). Paul often grounds exhortations on the assumption of Christian professors' normal state as realized (Rom 6:3-4) [ALFORD]. Regarding the Corinthian Church as the Passover "unleavened lump" or mass, he entreats them to correspond in fact with this their normal state. "For Christ our Passover (Exo 12:5-11, Exo 12:21-23; Joh 1:29) has been (English Version, "is") sacrificed for us"; that is, as the Jews began the days of unleavened bread with the slaying of the Passover lamb, so, Christ our Passover having been already slain, let there be no leaven of evil in you who are the "unleavened lump." Doubtless he alludes to the Passover which had been two or three weeks before kept by the Jewish Christians (Co1 16:8): the Gentile Christians probably also refraining from leavened bread at the love-feasts. Thus the Jewish Passover naturally gave place to our Christian Easter.

— Jamieson-Fausset-Brown Commentary (public domain)