Objection 1. It seems that a wicked priest cannot consecrate the Eucharist. For Jerome, commenting on Zephaniah 3:4, says: "The priests who perform the Eucharist, and who distribute our Lord's blood to the people, act wickedly against Christ's law, in deeming that the Eucharist is consecrated by a prayer rather than by a good life; and that only the solemn prayer is requisite, and not the priest's merits: of whom it is said: 'Let not the priest, in whatever defilement he may be, approach to offer oblations to the Lord'" (Leviticus 21:21, Septuagint). But the sinful priest, being defiled, has neither the life nor the merits befitting this sacrament. Therefore a sinful priest cannot consecrate the Eucharist.
Objection 2. Further, Damascene says (De Fide Orth. iv) that "the bread and wine are changed supernaturally into the body and blood of our Lord, by the coming of the Holy Ghost." But Pope Gelasius I says (Ep. ad Elphid., cf. Decret. i, q. 1): "How shall the Holy Spirit, when invoked, come for the consecration of the Divine Mystery, if the priest invoking him be proved full of guilty deeds?" Consequently, the Eucharist cannot be consecrated by a wicked priest.
Objection 3. 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."
On the contrary, Augustine (Paschasius) says (De Corp. Dom. xii): "Within the Catholic Church, in the mystery of the Lord's body and blood, nothing greater is done by a good priest, nothing less by an evil priest, because it is not by the merits of the consecrator that the sacrament is accomplished, but by the Creator's word, and by the power of the Holy Spirit."
I answer that, As was said above (1,3), the priest consecrates this sacrament not by his own power, but as the minister of Christ, in Whose person he consecrates this sacrament. But from the fact of being wicked he does not cease to be Christ's minister; because our Lord has good and wicked ministers or servants. Hence (Matthew 24:45) our Lord says: "Who, thinkest thou, is a faithful and wise servant?" and afterwards He adds: "But if that evil servant shall say in his heart," etc. And the Apostle (1 Corinthians 4:1) says: "Let a man so account of us as of the ministers of Christ"; and afterwards he adds: "I am not conscious to myself of anything; yet am I not hereby justified." He was therefore certain that he was Christ's minister; yet he was not certain that he was a just man. Consequently, a man can be Christ's minister even though he be not one of the just. And this belongs to Christ's excellence, Whom, as the true God, things both good and evil serve, since they are ordained by His providence for His glory. Hence it is evident that priests, even though they be not godly, but sinners, can consecrate the Eucharist.
Reply to Objection 1. In those words Jerome is condemning the error of priests who believed they could consecrate the Eucharist worthily, from the mere fact of being priests, even though they were sinners; and Jerome condemns this from the fact that persons defiled are forbidden to approach the altar; but this does not prevent the sacrifice, which they offer, from being a true sacrifice, if they do approach.
Reply to Objection 2. Previous to the words quoted, Pope Gelasius expresses himself as follows: "That most holy rite, which contains the Catholic discipline, claims for itself such reverence that no one may dare to approach it except with clean conscience." From this it is evident that his meaning is that the priest who is a sinner ought not to approach this sacrament. Hence when he resumes, "How shall the Holy Spirit come when summoned," it must be understood that He comes, not through the priest's merits, but through the power of Christ, Whose words the priest utters.
Reply to Objection 3. As the same action can be evil, inasmuch as it is done with a bad intention of the servant; and good from the good intention of the master; so the blessing of a sinful priest, inasmuch as he acts unworthily is deserving of a curse, and is reputed an infamy and a blasphemy, and not a prayer; whereas, inasmuch as it is pronounced in the person of Christ, it is holy and efficacious. Hence it is said with significance: "I will curse your blessings."
Source: Summa Theologica (New Advent)