Section — Wouters, Manual of Moral Theology
Section I. On the Person Offering
I. The principal minister is Christ the Lord. "It is the same one," says the Council of Trent, "now offering through the ministry of priests, who then offered Himself on the cross."1
II. The secondary minister is every and only priest. "If anyone says," the same Council of Trent says, "that by those words: Do this in commemoration of me, Christ did not institute the Apostles priests, or did not ordain that they and other priests should offer His body and blood, let him be anathema."2
III. In some sense all the faithful can be called ministers of the sacrifice of the Mass, namely insofar as they contribute something to the sacrifice. They do this insofar as they are members of the Church, in whose name the priest celebrates; insofar as they assist at the sacrifice and conform themselves to the intention of the principal and secondary minister; and insofar as they causally concur in the sacrifice, for example by giving a stipend or by serving the priest celebrating (St. Alphonsus, VI, 307).
Corollary I. Several priests can indeed offer the sacrifice of the Mass together, but this is illicit except in the Mass of the ordination of priests, and in the Mass of the consecration of bishops according to the Roman Pontifical (canon 803; n. 20).
Corollary II. A priest who is a stranger to a church, that is, not assigned to the church in which he asks to celebrate, can be admitted as follows:
If he shows authentic and still valid commendatory letters from his Ordinary if he is secular, or from his superior if he is religious, or from the Sacred Congregation for the Oriental Church if he is of an Oriental rite, he is to be admitted to the celebration of Mass, unless meanwhile it is clear that he has committed something on account of which he ought to be repelled from the celebration of Mass (canon 804, § 1).
If he lacks those letters, but his probity is very well known to the rector of the church, he can be admitted. But if he is unknown to the rector, he can still be admitted once or twice, provided that, clothed in ecclesiastical dress, he receives nothing, under any title, from the celebration from the church in which he sacrifices, and signs his name, office, and diocese in a special book (canon 804, § 2). It is said provided that he is clothed in ecclesiastical dress, that is, provided that the stranger, by the form of his dress, manifests that he is a priest.
Special norms concerning this matter, given by the local Ordinary, with the prescriptions of this canon saved, must be observed by all, even exempt religious, unless it concerns admitting religious to celebrate in a church of their own religious institute (canon 804, § 3).