Section — Slater, Manual of Moral Theology
Section II. The Faith and Holiness of the Minister
Neither faith nor the state of holiness and friendship with God is necessary in the minister for the validity of the sacraments which he confers. This is of faith, and it was defined by the Council of Trent. The sacraments do not depend for their effect on the good or bad dispositions of the minister, as they derive their efficacy from the institution and the merits of Christ. They produce their effect ex opere operato, not ex opere operantis. However, one who has been consecrated and deputed to be a dispenser of the mysteries of God is bound to fulfil his office in a worthy manner. Holy things must be treated holily. The minister acts in the name of Christ; he becomes the instrument of Christ for the sanctification of the souls of others by means of the sacraments; he would be greatly wanting in reverence and decency if, while engaged in so holy a task, his own conscience were stained with grievous sin. An enemy of God himself, he is guilty of great presumption in undertaking such holy functions. A consecrated minister who solemnly administers a sacrament while conscious of being in a state of mortal sin certainly sins grievously. The question whether a lay person who in case of necessity baptizes another or contracts marriage in the state of sin himself sins grievously, as being an unworthy minister of the sacrament, is much disputed among divines. Many weighty authorities excuse such a minister from grave sin because he is not under so strict an obligation to put himself in the state of grace before administering a sacrament as is one who has been set aside and consecrated to that office. All citizens are bound to defend their country when threatened, but there is a special obligation to do so incumbent on those who, like soldiers, have undertaken that duty. Similarly, all should indeed treat the sacraments with proper respect, but consecrated ministers are specially bound to do so while fulfilling their office. So that it.is a probable opinion that a lay person who baptizes in sin in a case of necessity, or one who marries and so ministers the sacrament in sin to the other party, does not thereby sin grievously. For the same reasons it is also probable that even a consecrated minister who while in sin administers Baptism privately in case of necessity does not sin mortally, for he then acts as a private person, not as a consecrated minister.
p. ii, 64
A priest who says Mass in the state of mortal sin is thereby guilty of several grievous sins. He celebrates Mass unworthily, he receives Holy Communion unworthily, and he gives himself the sacrament though he knows that he is unworthy to receive it. Some add a fourth sin, which is committed precisely by handling and administering the Blessed Sacrament in a state of sin. It is probable, however, that this last act, though wanting in due reverence, does not amount to a grievous sin, All the more is it a safe opinion that deacons and subdeacons who exercise their functions in a state of sin do not sin grievously by so doing, nor do Bishops and priests who in sin consecrate or bless pious objects, or preach the word of God.
Divines are not agreed whether a priest would commit one sin or as many sins as he administered sacraments unworthily who in a state of sin should hear many confessions or administer many Baptisms or other sacraments at the same time. If there were moral interruptions between the several sacraments, there would at least be as many sins as interruptions. But if there were no such moral interruption, it is a probable opinion that a priest who at one time administers a sacrament to many only commits one big mortal sin. The sin takes its unity from the fact that he exercises his office on one occasion unworthily, an office which he was consecrated to perform in a worthy manner. It is not, then, necessary for a priest who has sinned by hearing confessions in sin to say how many persons he has absolved; it will be sufficient if he states how often he has heard confessions in mortal sin.
It will be sufficient for a priest who is in sin to make an act of contrition before administering any of the other sacraments, but before saying Mass and receiving Holy Communion it is necessary for such a one to go to confession. The Council of Trent, commenting on the precept of St Paul, “ Let a man prove himself, and so let him eat of that bread,” says that the custom of the Church has always interpreted these words as implying that no one who is conscious of mortal sin ought to approach Holy Communion without sacramental confession, however contrite he may feel. It is, of course, advisable that this should be done before a minister who is in sin administers any of the sacraments, though it is only of strict obligation before saying Mass and receiving Holy Communion.
p. ii, 65–66