Objection 1: It seems that one may lawfully receive Communion from heretical, excommunicate, or even sinful priests, and to hear mass said by them. Because, as Augustine says (Contra Petilian. iii), "we should not avoid God's sacraments, whether they be given by a good man or by a wicked one." But priests, even if they be sinful, or heretics, or excommunicate, perform a valid sacrament. Therefore it seems that one ought not to refrain from receiving Communion at their hands, or from hearing their mass.
Obj. 2: Further, Christ's true body is figurative of His mystical body, as was said above (Q. 67, A. 2). But Christ's true body is consecrated by the priests mentioned above. Therefore it seems that whoever belongs to His mystical body can communicate in their sacrifices.
Obj. 3: Further, there are many sins graver than fornication. But it is not forbidden to hear the masses of priests who sin otherwise. Therefore, it ought not to be forbidden to hear the masses of priests guilty of this sin.
On the contrary, The Canon says (Dist. 32): "Let no one hear the mass of a priest whom he knows without doubt to have a concubine." Moreover, Gregory says (Dial. iii) that "the faithless father sent an Arian bishop to his son, for him to receive sacrilegiously the consecrated Communion at his hands. But, when the Arian bishop arrived, God's devoted servant rebuked him, as was right for him to do."
I answer that, As was said above (AA. 5, 7), heretical, schismatical, excommunicate, or even sinful priests, although they have the power to consecrate the Eucharist, yet they do not make a proper use of it; on the contrary, they sin by using it. But whoever communicates with another who is in sin, becomes a sharer in his sin. Hence we read in John's Second Canonical Epistle (11) that "He that saith unto him, God speed you, communicateth with his wicked works." Consequently, it is not lawful to receive Communion from them, or to assist at their mass.
Still there is a difference among the above, because heretics, schismatics, and excommunicates, have been forbidden, by the Church's sentence, to perform the Eucharistic rite. And therefore whoever hears their mass or receives the sacraments from them, commits sin. But not all who are sinners are debarred by the Church's sentence from using this power: and so, although suspended by the Divine sentence, yet they are not suspended in regard to others by any ecclesiastical sentence: consequently, until the Church's sentence is pronounced, it is lawful to receive Communion at their hands, and to hear their mass. Hence on 1 Cor. 5:11, "with such a one not so much as to eat," Augustine's gloss runs thus: "In saying this he was unwilling for a man to be judged by his fellow man on arbitrary suspicion, or even by usurped extraordinary judgment, but rather by God's law, according to the Church's ordering, whether he confess of his own accord, or whether he be accused and convicted."
Reply Obj. 1: By refusing to hear the masses of such priests, or to receive Communion from them, we are not shunning God's sacraments; on the contrary, by so doing we are giving them honor (hence a host consecrated by such priests is to be adored, and if it be reserved, it can be consumed by a lawful priest): but what we shun is the sin of the unworthy ministers.
Reply Obj. 2: The unity of the mystical body is the fruit of the true body received. But those who receive or minister unworthily, are deprived of the fruit, as was said above (A. 7; Q. 80, A. 4). And therefore, those who belong to the unity of the Faith are not to receive the sacrament from their dispensing.
Reply Obj. 3: Although fornication is not graver than other sins, yet men are more prone to it, owing to fleshly concupiscence. Consequently, this sin is specially inhibited to priests by the Church, lest anyone hear the mass of one living in concubinage. However, this is to be understood of one who is notorious, either from being convicted and sentenced, or from having acknowledged his guilt in legal form, or from it being impossible to conceal his guilt by any subterfuge.