Thomas Aquinas

“ Hence in the early Church, when men had to be enticed to the faith by means of outward signs (thus the gift of the Holy Ghost was shown openly by a visible sign) , so too excommunication was evidenced by a person being troubled in his body by the devil. Nor is it unreasonable that one, for whom there is still hope, be given over to the enemy, for he is surrendered, not unto damnation, but unto correction, since the Church has the power to rescue him from the hands of the enemy, whenever he is willing. ”

“ When a man is corrected by being punished, evil is not rendered to him, but good: since punishments are remedies, as stated above.
Art. 3 - Whether an excommunication unjustly pronounced has any effect? Objection 1: It would seem that an excommunication which is pronounced unjustly has no effect at all. Because excommunication deprives a man of the protection and grace of God, which cannot be forfeited unjustly. Therefore excommunication has no effect if it be unjustly pronounced. ”

“ In like manner prayers may be offered up for those who are excommunicated, but not among the prayers that are said for the members of the Church. Yet they do not receive the fruit so long as they remain under the excommunication, but prayers are said for them that they may receive the spirit of repentance, so that they may be loosed from excommunication.
Reply to Objection 2: One man's prayers profit another in so far as they can reach to him. Now the action of one man may reach to another in two ways. ”

“ Further, no man should be given into the hands of his enemies, unless there be no hope for him. Now by excommunication a man is given into the hands of Satan, as is clear from 1 Cor. 5:5. Since then we should never give up hope about anyone in this life, the Church should not excommunicate anyone. ”

“ These different manners of communion are signified in the verse which declares that those who are excommunicate are deprived of---
"os, orare, vale, communio, mensa."
"Os," i.e. we must not give them tokens of goodwill; "orare," i.e. we must not pray with them; "vale," we must not give them marks of respect; "communio," i.e. we must not communicate with them in the sacraments; "mensa," i.e. we must not take meals with them. ”