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.
Not ... old leaven - of our unconverted state.
Malice - the opposite of "sincerity," which allows no leaven of evil to be mixed up with good (Matthew 16:6).
Wickedness - the opposite of "truth," which allows not evil to be mistaken for good. "Malice" х kakia (G2549)] means the evil habit of mind; "wickedness" х poneeria (G4189)] its outcoming in word and deed. "Sincerity" х eilikrineia (G1505)] expresses literally a thing which, examined by sunlight, is found unadulterated.