Thomas Aquinas

“ By reason of the power of the Holy Ghost, Who communicates to each one the blessings of Christ's members on account of their being united in charity, the private blessing in the mass of a good priest is fruitful to others. But the private evil of one man cannot hurt another, except the latter, in some way, consent, as Augustine says (Contra Parmen. ii) .
Art. 7 - Whether heretics, schismatics, and excommunicated persons can consecrate? Objection 1: It seems that heretics, schismatics, and excommunicated persons are not able to consecrate the Eucharist. ”

“ 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.
Art. 10 - Whether it is lawful for a priest to refrain entirely from consecrating the Eucharist? ”

“ But in like fashion, if the degraded man be restored, he has not to be ordained over again. Consequently, he has not lost the power of consecrating, and so the degraded priest can perform this sacrament.
I answer that, The power of consecrating the Eucharist belongs to the character of the priestly order. But every character is indelible, because it is given with a kind of consecration, as was said above ( [4690] Q [63] , A [5] ) , just as the consecrations of all other things are perpetual, and cannot be lost or repeated. ”

“ I answer that, There are two things to be considered in the mass. namely, the sacrament itself, which is the chief thing; and the prayers which are offered up in the mass for the quick and the dead. So far as the mass itself is concerned, the mass of a wicked priest is not of less value than that of a good priest, because the same sacrifice is offered by both. ”

“ 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. ”

“ The priest, in reciting the prayers of the mass, speaks instead of the Church, in whose unity he remains; but in consecrating the sacrament he speaks as in the person of Christ, Whose place he holds by the power of his orders. Consequently, if a priest severed from the unity of the Church celebrates mass, not having lost the power of order, he consecrates Christ's true body and blood ”

“ Further, this sacrament is consecrated by the priest's blessing. But a sinful priest's blessing is not efficacious for consecrating this sacrament, since it is written (Malachi 2:2) : "I will curse your blessings." Again, Dionysius says in his Epistle (viii) to the monk Demophilus: "He who is not enlightened has completely fallen away from the priestly order; and I wonder that such a man dare to employ his hands in priestly actions, and in the person of Christ to utter, over the Divine symbols, his unclean infamies, for I will not call them prayers." ”

“ 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 to Objection 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 [4693] (A [7] ; Q [80] , A [4] ) . ”

“ As the canonical precepts do not permit them who are feeble in body to approach the priesthood, so if anyone be disabled when once in that state, he cannot lose that he received at the time he was well. ”

“ But it sometimes happens that those who are already ordained as priests incur defects whereby they are hindered from celebrating, such as leprosy or epilepsy, or the like. Consequently, it does not appear that priests are bound to celebrate. ”