Objection 1. It seems that the mass of a sinful priest is not of less worth than that of a good priest. For Pope Gregory says in the Register: "Alas, into what a great snare they fall who believe that the Divine and hidden mysteries can be sanctified more by some than by others; since it is the one and the same Holy Ghost Who hallows those mysteries in a hidden and invisible manner." But these hidden mysteries are celebrated in the mass. Therefore the mass of a sinful priest is not of less value than the mass of a good priest.
Objection 2. Further, as Baptism is conferred by a minister through the power of Christ Who baptizes, so likewise this sacrament is consecrated in the person of Christ. But Baptism is no better when conferred by a better priest, as was said above (64, 1, ad 2). Therefore neither is a mass the better, which is celebrated by a better priest.
Objection 3. Further, as the merits of priests differ in the point of being good and better, so they likewise differ in the point of being good and bad. Consequently, if the mass of a better priest be itself better, it follows that the mass of a bad priest must be bad. Now this is unreasonable, because the malice of the ministers cannot affect Christ's mysteries, as Augustine says in his work on Baptism (Contra Donat. xii). Therefore neither is the mass of a better priest the better.
On the contrary, It is stated in Decretal i, q. 1: "The worthier the priest, the sooner is he heard in the needs for which he prays."
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.
Again, the prayer put up in the mass can be considered in two respects: first of all, in so far as it has its efficacy from the devotion of the priest interceding, and in this respect there is no doubt but that the mass of the better priest is the more fruitful. In another respect, inasmuch as the prayer is said by the priest in the mass in the place of the entire Church, of which the priest is the minister; and this ministry remains even in sinful men, as was said above (Article 5) in regard to Christ's ministry. Hence, in this respect the prayer even of the sinful priest is fruitful, not only that which he utters in the mass, but likewise all those he recites in the ecclesiastical offices, wherein he takes the place of the Church. on the other hand, his private prayers are not fruitful, according to Proverbs 28:9: "He that turneth away his ears from hearing the law, his prayer shall be an abomination."
Reply to Objection 1. Gregory is speaking there of the holiness of the Divine sacrament.
Reply to Objection 2. In the sacrament of Baptism solemn prayers are not made for all the faithful, as in the mass; therefore there is no parallel in this respect. There is, however, a resemblance as to the effect of the sacrament.
Reply to Objection 3. 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).
Source: Summa Theologica (New Advent)