I answer that, A sinful action consists in this, that a man "fails to act as he ought to," as the Philosopher explains (Ethic. ii). Now it has been said (A. 5, ad 3) that it is fitting for the ministers of sacraments to be righteous; because ministers should be like unto their Lord, according to Lev. 19:2: "Be ye holy, because I . . . am holy"; and Ecclus. 10:2: "As the judge of the people is himself, so also are his ministers." Consequently, there can be no doubt that the wicked sin by exercising the ministry of God and the Church, by conferring the sacraments. And since this sin pertains to irreverence towards God and the contamination of holy things, as far as the man who sins is concerned, although holy things in themselves cannot be contaminated; it follows that such a sin is mortal in its genus.
Reply Obj. 1: Works of charity are not made holy by some process of consecration, but they belong to the holiness of righteousness, as being in a way parts of righteousness. Consequently, when a man shows himself as a minister of God, by doing works of charity, if he be righteous, he will be made yet holier; but if he be a sinner, he is thereby disposed to holiness. On the other hand, the sacraments are holy in themselves owing to their mystical consecration. Wherefore the holiness of righteousness is required in the minister, that he may be suitable for his ministry: for which reason he acts unbecomingly and sins, if while in a state of sin he attempts to fulfil that ministry.
Reply Obj. 2: He who approaches a sacrament, receives it from a minister of the Church, not because he is such and such a man, but because he is a minister of the Church. Consequently, as long as the latter is tolerated in the ministry, he that receives a sacrament from him, does not communicate in his sin, but communicates with the Church from whom he has his ministry. But if the Church, by degrading, excommunicating, or suspending him, does not tolerate him in the ministry, he that receives a sacrament from him sins, because he communicates in his sin.
Reply Obj. 3: A man who is in mortal sin is not perplexed simply, if by reason of his office it be his bounden duty to minister sacraments; because he can repent of his sin and so minister lawfully. But there is nothing unreasonable in his being perplexed, if we suppose that he wishes to remain in sin.
However, in a case of necessity when even a lay person might baptize, he would not sin in baptizing. For it is clear that then he does not exercise the ministry of the Church, but comes to the aid of one who is in need of his services. It is not so with the other sacraments, which are not so necessary as baptism, as we shall show further on (Q. 65, AA. 3, 4; Q. 62, A. 3). _______________________
SEVENTH ARTICLE [III, Q. 64, Art. 7]
Whether Angels Can Administer Sacraments?
Objection 1: It seems that angels can administer sacraments. Because a higher minister can do whatever the lower can; thus a priest can do whatever a deacon can: but not conversely. But angels are higher ministers in the hierarchical order than any men whatsoever, as Dionysius says (Coel. Hier. ix). Therefore, since men can be ministers of sacraments, it seems that much more can angels be.
Obj. 2: Further, in heaven holy men are likened to the angels (Matt. 22:30). But some holy men, when in heaven, can be ministers of the sacraments; since the sacramental character is indelible, as stated above (Q. 63, A. 5). Therefore it seems that angels too can be ministers of sacraments.
Obj. 3: Further, as stated above (Q. 8, A. 7), the devil is head of the wicked, and the wicked are his members. But sacraments can be administered by the wicked. Therefore it seems that they can be administered even by demons.