Isolation And Cure

Bonus itaque doctor dum promittit alterum de duobus, utrumque donavit. Venit in virga, quia a communione sacra convictum removit. Et bene dicitur tradi satanae, qui separantur a Christi corpore. Venit etiam in charitate, spirituque mansuetudinis, vel quia sic traididit, ut spiritum ejus salvum faceret, vel quia eum quem ante sequestraverat postea sacramentis reddidit. Nam et sequestrari oportet graviter lapsum, ne modicum fermentum totam massam corrumpat: et expurgandum est vetus fermentum, vel in singulis vetus homo, hoc est, exterior homo cum actibus suis, vel in populo inveteratus peccatis, vitiisque concretus. Et bene dixit expurgandum, non projiciendum: quod enim expurgatur, non totum judicatur inutile; ideo enim purgatur, ut utile ab inutili separetur: quod autem projicitur, nihil in se utile habere creditur. Sanctus Ambrosius Mediolanensis, De Paenitentia, Liber I, Caput XV Source: Migne PL 16.490a-b

So the good teacher, the Apostle, while he promised one of two things, gifted both. He came with a rod, for he separated the guilty man from the holy community, and well is it said that they are given over to Satan who are separated from the body of Christ. But he came also with charity and with the spirit of meekness, whether because he so delivered another up so as to save his soul, or because he afterwards restored him to the sacraments from which he had separated. For it is necessary to separate one who has grievously fallen lest a little leaven corrupt the whole lump. And the old leaven must be purged out, 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 that which is worthless, but that which is cast forth is considered to have in itself nothing of value. Saint Ambrose of Milan, On Penitence, Book 1, Chapter 15