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.
All Commentaries on 1 Corinthians 5:8 Go To 1 Corinthians 5
John Chrysostom
AD 407
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.