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
Ambrosiaster
AD 400
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.