Let us keep the feast - Margin, “Holy day” ἑορτάζωμεν heortazōmen. This is language drawn from the paschal feast, and is used by Paul frequently to carry out and apply his illustration. It does not mean literally the paschal supper here - for that had ceased to be observed by Christians - nor the Lord’s Supper particularly; but the sense is “As the Jews when they celebrated the paschal supper, on the slaying and sacrifice of the paschal lamb, put away all leaven - as emblematic of sin - so let us, in the slaying of our sacrifice, and in all the duties, institutions and events consequent thereon, put away all wickedness from our hearts as individuals, and from our societies and churches. Let us engage in the service of God putting away by all evil.”
Not with the old leaven - Not under the influence, or in the indulgence of the feelings of corrupt and unrenewed human nature - The word “leaven” is very expressive of that former or “old” condition, and denotes the corrupt and corrupting passions of our nature before it is renewed.
The leaven of malice - Of unkindness and evil - which would diffuse itself, and pervade the mass of Christians. The word “malice” (κακίας kakias) denotes “evil” in general.
And wickedness - Sin; evil. There is a particular reference here to the case of the incestuous person. Paul means that all wickedness should be put away from those who had been saved by the sacrifice of their “Passover,” Christ; and, therefore, this sin in a special manner.
But with the unleavened bread ... - That is, with sincerity and truth. Let us be sincere, and true, and faithful; as the Jews partook of bread unleavened, which was emblematic of purity, so let us be sincere and true. It is implied here that this could not be done unless they would put away the incestuous person - No Christians can have, or give evidence of sincerity, who are not willing to put away all sin.