1 Corinthians 5:7-8
“Purge out the old leaven,” that is, this evil one. Not that he speaks concerning this one only; rather he glances at others with him. For, “the old leaven” is not fornication only, but also sin of every kind. And he said not, “purge,” but “purge out;” “cleanse with accuracy so that there be not so much as a remnant nor a shadow of that sort.” In saying then, “purge out,” he signifies that there was still iniquity among them. But in saying, “that you may be a new lump, even as you are unleavened,” he affirms and declares that not over very many was the wickedness prevailing. But though he says, “as you are unleavened,” he means it not as a fact that all were clean, but as to what sort of people you ought to be.
6. “For our Passover also has 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 has not come to pass that is good? The Son of God was made man for you; He freed you from death; and called you to a kingdom. You therefore who hast obtained and art still obtaining such things, how can it be less than your duty to “keep the feast” all your 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 says 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 has been made, a spiritual marriage. For, “the kingdom of Heaven,” says 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.
7. 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 should you 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 drove us out, who before held us forcibly; they did not suffer us so much as to leaven our dough.” But if a man asks 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.
Again, should you ask the Jew why he expels all leaven from all his borders; here he will even be silent and will not so much as state any reason. And this is because, although some indeed of the circumstances were both types of things to come, and also due to things then happening; yet others were not so, that the Jews might not deal deceitfully; that they might not abide in the shadow. For tell me, what is the meaning of the Lamb's being a “Male,” and “Unblemished,” and a “year old,” and of, “a bone shall not be broken?” and what means the command to call the neighbors also, and that it should be eaten “standing” and “in the evening;” or the fortifying the house with blood? He will have nothing else to say but over and over all about Egypt. But I can tell you the meaning both of the Blood, and of the Evening, and the Eating all together, and of the rule that all should be standing.
8. But first let us explain why the leaven is cast out of all their borders. What then is the hidden meaning? The believer must be freed from all iniquity. For as among them he perishes with whomsoever is found old leaven, so also with us wheresoever is found iniquity: since of course the punishment being so great in that which is a shadow, in our case it cannot choose but be much greater. For if they so carefully clear their houses of leaven, and pry into mouse-holes; much more ought we to search through the soul so as to cast out every unclean thought.
This however was done by them of late; but now no longer. For every where there is leaven, where a Jew is found. For it is in the midst of cities that the feast of unleavened bread is kept: a thing which is now rather a game at play than a law. For since the Truth has come, the Types have no longer any place.
So that by means of this example also he mightily drives the fornicator out of the Church. For, says he, so far from his presence profiting, he even does harm, injuring the common estate of the body. For one knows not whence is the evil savor while the corrupt part is concealed, and so one imputes it to the whole. Wherefore he urges upon them strongly to “purge out the leaven, that you may be,” says he, “a new lump, even as you are unleavened.”
“For our Passover has been sacrificed for us even Christ.” He said not, has died, but more in point to the subject in hand, “has been sacrificed.” Seek not then unleavened bread of this kind, since neither have you a lamb of the same kind. Seek not leaven of this description, seeing that your unleavened bread is not such as this.
9. Thus, in the case of material leaven, the unleavened might become leavened, but never the reverse; whereas here there is a chance of the direct contrary occuring. This however he has not plainly declared: and observe his good sense. In the former Epistle he gives the fornicator no hope of return, but orders that his whole life should be spent in repentance, lest he should make him less energetic through the promise. For he said not, “Deliver him up to Satan,” that having repented he might be commended again unto the Church. But what says he? “That he may be saved in the last day.” For he conducts him on unto that time in order to make him full of anxiety. And what favors he intended him after the repentance, he reveals not, imitating his own Master. For as God says, “Yet three days, and Nineveh shall be overthrown,” and added not, “but if she repent she shall be saved:” so also he did not say here, “But if he repent worthily, we will 'confirm our love towards him.'”. But he waits for him to do the work that so he may then receive the favor. For if he had said this at the beginning he might have set him free from the fear. Wherefore he not only does not so, but by the instance of leaven allows him not even a hope of return, but reserves him unto that day: “Purge out (so he says) the old leaven;” and, “let us not keep the feast with old leaven.” But as soon as he had repented, he brought him in again with all earnestness.
Source: Homilies on First Corinthians (New Advent)