Ambrosiaster : 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 John Chrysostom : "For our Passover also hath been sacrificed for us, even Christ; wherefore let us keep the feast: not with old leaven, nor with the leaven of malice and wickedness, but with the unleavened bread of sincerity and truth." So also Christ called His doctrine Leaven. And further he himself dwells upon the metaphor, reminding them of an ancient history, and of the Passover and unleavened bread, and of their blessings both then and now, and their punishments and their plagues. It is festival, therefore, the whole time in which we live. For though he said, "Let us keep the feast," not with a view to the presence of the Passover or of Pentecost did he say it; but as pointing out that the whole of time is a festival unto Christians, because of the excellency of the good things which have been given. For what hath not come to pass that is good? The Son of God was made man for thee; He freed thee from death; and called thee to a kingdom. Thou therefore who hast obtained and art still obtaining such things, how can it be less than thy duty to "keep the feast" all thy life? Let no one then be downcast about poverty, and disease, and craft of enemies. For it is a festival, even the whole of our time. Wherefore saith Paul, "Rejoice in the Lord always; again I say, Rejoice." Upon the festival days no one puts on filthy garments. Neither then let us do so. For a marriage hath been made, a spiritual marriage. For, "the kingdom of Heaven," saith He, "is likened unto a certain king which would make a marriage feast for his son." Now where it is a king making a marriage, and a marriage for his son, what can be greater than this feast? Let no one then enter in clad in rags. Not about garments is our discourse but about unclean actions. For if where all wore bright apparel one alone, being found at the marriage in filthy garments, was cast out with dishonor, consider how great strictness and purity the entrance into that marriage feast requires. However, not on this account only does he remind them of the "unleavened bread," but also to point out the affinity of the Old Testament with the New; and to point out also that it was impossible, after the "unleavened bread," again to enter into Egypt; but if any one chose to return, he would suffer the same things as did they. For those things were a shadow of these; however obstinate the Jew may be. Wherefore shouldest thou enquire of him, he will speak, no great thing, rather it is great which he will speak of, but nothing like what we speak of: because he knows not the truth. For he for his part will say, "the Egyptians who detained us were so changed by the Almighty that they themselves urged and drave us out, who before held us forcibly; they did not suffer us so much as to leaven our dough." But if a man asketh me, he shall hear not of Egypt nor of Pharaoh; but of our deliverance from the deceit of demons and the darkness of the devil: not of Moses but of the Son of God; not of a Red Sea but of a Baptism overflowing with ten thousand blessings, where the "old man" is drowned. — Homily on 1 Corinthians 15