1 Corinthians 5 v 7-8


1 Corinthians 5:7-8
(7)  Purge out therefore the old leaven, that ye may be a new lump, as ye are unleavened. For even Christ our passover is sacrificed for us:
(8)  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.


5:7-8. As the literal yeast was removed from the house during the Festival of Unleavened Bread (Ex. 12:15-20; 13:1-10), so that which it illustrated, sin, was to be removed from the house of God, the local church, during its “Festival of Unleavened Bread,” a continual observance for a Christian who has found in Christ’s death on the cross the once-for-all sacrifice of the Passover Lamb (cf. John 1:29; Heb. 10:10, 14). This was nowhere more true than in the celebration which commemorated that sacrificial act, the Lord’s Supper, the quintessential act of fellowship for Christians. Probably Paul meant to exclude the unrepentant Christian from this meal in particular.

Excerpt from:
Walvoord, J. F., Zuck, R. B., & Dallas Theological Seminary. (1983-c1985). 
The Bible knowledge commentary : An Exposition of the Scriptures. 
Wheaton, IL: Victor Books.