Objection 1. It seems that wicked men do not sin in administering the sacraments. For just as men serve God in the sacraments, so do they serve Him in works of charity; whence it is written (Hebrews 13:16): "Do not forget to do good and to impart, for by such sacrifices God's favor is obtained." But the wicked do not sin in serving God by works of charity: indeed, they should be persuaded to do so, according to Daniel 4:24: "Let my counsel be acceptable" to the king; "Redeem thou thy sins with alms." Therefore it seems that wicked men do not sin in administering the sacraments.
Objection 2. Further, whoever co-operates with another in his sin, is also guilty of sin, according to Romans 1:32: "He is [Vulgate: 'They are'] worthy of death; not only he that commits the sin, but also he who consents to them that do them." But if wicked ministers sin in administering sacraments, those who receive sacraments from them, co-operate in their sin. Therefore they would sin also; which seems unreasonable.
Objection 3. Further, it seems that no one should act when in doubt, for thus man would be driven to despair, as being unable to avoid sin. But if the wicked were to sin in administering sacraments, they would be in a state of perplexity: since sometimes they would sin also if they did not administer sacraments; for instance, when by reason of their office it is their bounden duty to do so; for it is written (1 Corinthians 9:16): "For a necessity lieth upon me: Woe is unto me if I preach not the gospel." Sometimes also on account of some danger; for instance, if a child in danger of death be brought to a sinner for baptism. Therefore it seems that the wicked do not sin in administering the sacraments.
On the contrary, Dionysius says (Eccl. Hier. i) that "it is wrong for the wicked even to touch the symbols," i.e. the sacramental signs. And he says in the epistle to Demophilus: "It seems presumptuous for such a man," i.e. a sinner, "to lay hands on priestly things; he is neither afraid nor ashamed, all unworthy that he is, to take part in Divine things, with the thought that God does not see what he sees in himself: he thinks, by false pretenses, to cheat Him Whom he calls his Father; he dares to utter, in the person of Christ, words polluted by his infamy, I will not call them prayers, over the Divine symbols."
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 (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 Leviticus 19:2: "Be ye holy, because I... am holy"; and Sirach 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 to Objection 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 to Objection 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 to Objection 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 (65, 3,4; 62, 3).
Source: Summa Theologica (New Advent)