Objection 1. It seems that faith is required of necessity in the minister of a sacrament. For, as stated above (Article 8), the intention of the minister is necessary for the validity of a sacrament. But "faith directs in intention" as Augustine says against Julian (In Psalm xxxi, cf. Contra Julian iv). Therefore, if the minister is without the true faith, the sacrament is invalid.
Objection 2. Further, if a minister of the Church has not the true faith, it seems that he is a heretic. But heretics, seemingly, cannot confer sacraments. For Cyprian says in an epistle against heretics (lxxiii): "Everything whatsoever heretics do, is carnal, void and counterfeit, so that nothing that they do should receive our approval." And Pope Leo says in his epistle to Leo Augustus (clvi): "It is a matter of notoriety that the light of all the heavenly sacraments is extinguished in the see of Alexandria, by an act of dire and senseless cruelty. The sacrifice is no longer offered, the chrism is no longer consecrated, all the mysteries of religion have fled at the touch of the parricide hands of ungodly men." Therefore a sacrament requires of necessity that the minister should have the true faith.
Objection 3. Further, those who have not the true faith seem to be separated from the Church by excommunication: for it is written in the second canonical epistle of John (10): "If any man come to you, and bring not this doctrine, receive him not into the house, nor say to him; God speed you": and (Titus 3:10): "A man that is a heretic, after the first and second admonition avoid." But it seems that an excommunicate cannot confer a sacrament of the Church: since he is separated from the Church, to whose ministry the dispensation of the sacraments belongs. Therefore a sacrament requires of necessity that the minister should have the true faith.
On the contrary, Augustine says against the Donatist Petilian: "Remember that the evil lives of wicked men are not prejudicial to God's sacraments, by rendering them either invalid or less holy."
I answer that, As stated above (Article 5), since the minister works instrumentally in the sacraments, he acts not by his own but by Christ's power. Now just as charity belongs to a man's own power so also does faith. Wherefore, just as the validity of a sacrament does not require that the minister should have charity, and even sinners can confer sacraments, as stated above (Article 5); so neither is it necessary that he should have faith, and even an unbeliever can confer a true sacrament, provided that the other essentials be there.
Reply to Objection 1. It may happen that a man's faith is defective in regard to something else, and not in regard to the reality of the sacrament which he confers: for instance, he may believe that it is unlawful to swear in any case whatever, and yet he may believe that baptism is an efficient cause of salvation. And thus such unbelief does not hinder the intention of conferring the sacrament. But if his faith be defective in regard to the very sacrament that he confers, although he believe that no inward effect is caused by the thing done outwardly, yet he does know that the Catholic Church intends to confer a sacrament by that which is outwardly done. Wherefore, his unbelief notwithstanding, he can intend to do what the Church does, albeit he esteem it to be nothing. And such an intention suffices for a sacrament: because as stated above (8, ad 2) the minister of a sacrament acts in the person of the Church by whose faith any defect in the minister's faith is made good.
Reply to Objection 2. Some heretics in conferring sacraments do not observe the form prescribed by the Church: and these confer neither the sacrament nor the reality of the sacrament. But some do observe the form prescribed by the Church: and these confer indeed the sacrament but not the reality. I say this in the supposition that they are outwardly cut off from the Church; because from the very fact that anyone receives the sacraments from them, he sins; and consequently is hindered from receiving the effect of the sacrament. Wherefore Augustine (Fulgentius, De Fide ad Pet.) says: "Be well assured and have no doubt whatever that those who are baptized outside the Church, unless they come back to the Church, will reap disaster from their Baptism." In this sense Pope Leo says that "the light of the sacraments was extinguished in the Church of Alexandria"; viz. in regard to the reality of the sacrament, not as to the sacrament itself.
Cyprian, however, thought that heretics do not confer even the sacrament: but in this respect we do not follow his opinion. Hence Augustine says (De unico Baptismo xiii): "Though the martyr Cyprian refused to recognize Baptism conferred by heretics or schismatics, yet so great are his merits, culminating in the crown of martyrdom, that the light of his charity dispels the darkness of his fault, and if anything needed pruning, the sickle of his passion cut it off."
Reply to Objection 3. The power of administering the sacraments belongs to the spiritual character which is indelible, as explained above (63, 3). Consequently, if a man be suspended by the Church, or excommunicated or degraded, he does not lose the power of conferring sacraments, but the permission to use this power. Wherefore he does indeed confer the sacrament, but he sins in so doing. He also sins that receives a sacrament from such a man: so that he does not receive the reality of the sacrament, unless ignorance excuses him.
Source: Summa Theologica (New Advent)