Returning from this digression, St. Ambrose explains what is the meaning of St. Paul where he speaks of coming “with a rod or in the spirit of meekness.” One who has grievously fallen is to be separated, but to be again restored to religious privileges when he has sufficiently repented. The old leaven is purged out when the hardness of the letter is tempered by the meal of a milder interpretation. All should be sprinkled with the Church's meal and fed with the food of charity, lest they become like that envious elder brother, whose example is followed by the Novatians.
78. That faithful teacher, having promised one of two things, gave each. He came with a rod, for he separated the guilty man from the holy fellowship. And well is he said to be delivered to Satan who is separated from the body of Christ. But he came in love and with the spirit of meekness, whether because he so delivered him up as to save his soul, or because he afterwards restored to the sacraments him whom he had before separated.
79. For it is needful to separate one who has grievously fallen, lest a little leaven corrupt the whole lump. And the old leaven must be purged out, or the old man in each person; that is, the outward man and his deeds, he who among the people has grown old in sin and hardened in vices. And well did he say purged, not cast forth, for what is purged is not considered wholly valueless, for to this end is it purged, that what is of value be separated from the worthless, but that which is cast forth is considered to have in itself nothing of value.
80. The Apostle then judged that the sinner should then at once be restored to the heavenly sacraments if he himself wished to be cleansed. And well is it said “Purge,” for he is purged as by certain things done by the whole people, and is washed in the tears of the multitude, and redeemed from sin by the weeping of the multitude, and is purged in the inner man. For Christ granted to His Church that one should be redeemed by means of all, as she herself was found worthy of the coming of the Lord Jesus, in order that through One all might be redeemed.
81. This is Paul's meaning which the words make more obscure. Let us consider the exact words of the Apostle: “Purge out,” says he, “the old leaven, that you may be a new lump, even as you are unleavened.” Either that the whole Church takes up the burden of the sinner, with whom she has to suffer in weeping and prayer and pain, and, as it were, covers herself with his leaven, in order that by means of all that which is to be done away in the individual doing penance may be purged by a kind of contribution and commixture of compassion and mercy offered with manly vigor. Or one may understand it as that woman in the Gospel teaches us, who is a type of the Church, when she hid the leaven in her meal, till all was leavened, and the whole could be used as pure.
82. The Lord taught me in the Gospel what leaven is when He said: “Do you not understand that I said not concerning bread, Beware of the leaven of the Pharisees and Sadducees?” Then, it is said, they understood that He spoke not of bread, but that they should beware of the doctrine of the Pharisees and Sadducees. This leaven, then— that is, the doctrine of the Pharisees and the contentiousness of the Sadducees— the Church hides in her meal, when she softened the hard letter of the Law by a spiritual interpretation, and ground it as it were in the mill of her explanations, bringing out as it were from the husks of the letter the inner secrets of the mysteries, and setting forth the belief in the Resurrection, wherein the mercy of God is proclaimed, and wherein it is believed that the life of those who are dead is restored.
83. Now this comparison seems to be not unfitly brought forward in this place, since the kingdom of heaven is redemption from sin, and therefore we all, both bad and good, are mingled with the meal of the Church that we all may be a new lump. But that no one may be afraid that an admixture of evil leaven might injure the lump, the Apostle said: “That you may be a new lump, even as you are unleavened;” that is to say, This mixture will render you again such, as in the pure integrity of your innocence. If we thus have compassion, we are not stained with the sins of others, but we gain the restoration of another to the increase of our own grace, so that our integrity remains as it was. And therefore he adds: “For Christ our Passover is sacrificed for us;” that is, the Passion of the Lord profited all, and gave redemption to sinners who repented of the sins they had committed.
84. Let us then keep the feast on good food, doing penance yet joyful in our redemption, for no food is sweeter than kindness and gentleness. Let no envy towards the sinner who is saved be mingled with our feasts and joy, lest that envious brother, as is set forth in the Gospel, exclude himself from the house of his Father, because he grieved at the reception of his brother, at whose lasting exile he was wont to rejoice.
85. And you Novatians cannot deny that you are like him, who, as you say, do not come together to the Church because by penance a hope of return had been given to those who had lapsed. But this is only a pretence, for Novatian contrived his schism through grief at his loss of the episcopal office.
86. But do you not understand that the Apostle also prophesied of you and says to you: “And you are puffed up and did not rather mourn, that he who did this deed might be taken away from among you”? He is, then, wholly taken away when his sin is done away, but the Apostle does not say that the sinner is to be shut out of the Church who counsels his cleansing.
Source: On Repentance (New Advent)