V. Avoid all dealings with the heretics, but intercede with God for them
To confirm your hearts therefore, dearly-beloved, in the Faith and Truth, let today's festival help you all, and let the catholic confession be fortified by the testimony of the manifestation of the Saviour's infancy, while we anathematize the blasphemy of those who deny the flesh of our nature in Christ: about which the blessed Apostle John has forewarned us in no doubtful utterance, saying, “every spirit which confesses Christ Jesus to have come in the flesh is of God: and every spirit which destroys Jesus is not of God, and this is Antichrist.”
Consequently let no Christian have anything in common with men of this kind, let him have no alliance or intercourse with such. Let it advantage the whole Church that many of them in the mercy of God have been discovered, and that their own confession has disclosed how sacrilegious their lives were. Let no one be deceived by their discriminations between food and food, by their soiled raiment, by their pale faces. Fasts are not holy which proceed not on the principle of abstinence but with deceitful design.
Let this be the end of their harming the unwary, and deluding the ignorant; henceforth no one's fall shall be excusable: no longer must he be held simple but extremely worthless and perverse who hereafter shall be found entangled in detestable error. A practice countenanced by the Church and Divinely instituted, not only do we not forbid, we even incite you to, that you should supplicate the Lord even for such: since we also with tears and mourning feel pity for the ruins of cheated souls, carrying out the Apostles' example of loving-kindness, so as to be weak with those that are weak and to “weep with those that weep.”
For we hope that God's mercy can be won by the many tears and due amendment of the fallen: because so long as life remains in the body no man's restoration must be despaired of, but the reform of all desired with the Lord's help, “who raises up them that are crushed, looses them that are chained, gives light to the blind:” to whom is honour and glory for ever and ever. Amen.
Source: Sermons (New Advent)