Objection 1: It seems that it does not belong to a priest to catechize and exorcize the person to be baptized. For it belongs to the office of ministers to operate on the unclean, as Dionysius says (Eccl. Hier. v). But catechumens who are instructed by catechism, and "energumens" who are cleansed by exorcism, are counted among the unclean, as Dionysius says in the same place. Therefore to catechize and to exorcize do not belong to the office of the priests, but rather to that of the ministers.
Obj. 2: Further, catechumens are instructed in the Faith by the Holy Scripture which is read in the church by ministers: for just as the Old Testament is recited by the Readers, so the New Testament is read by the Deacons and Subdeacons. And thus it belongs to the ministers to catechize. In like manner it belongs, seemingly, to the ministers to exorcize. For Isidore says (Epist. ad Ludifred.): "The exorcist should know the exorcisms by heart, and impose his hands on the energumens and catechumens during the exorcism." Therefore it belongs not to the priestly office to catechize and exorcize.
Obj. 3: Further, "to catechize" is the same as "to teach," and this is the same as "to perfect." Now this belongs to the office of a bishop, as Dionysius says (Eccl. Hier. v). Therefore it does not belong to the priestly office.
On the contrary, Pope Nicolas I says: "The catechizing of those who are to be baptized can be undertaken by the priests attached to each church." And Gregory says (Hom. xxix super Ezech.): "When priests place their hands on believers for the grace of exorcism, what else do they but cast out the devils?"
I answer that, The minister compared to the priest, is as a secondary and instrumental agent to the principal agent: as is implied in the very word "minister." Now the secondary agent does nothing without the principal agent in operating. And the more mighty the operation, so much the mightier instruments does the principal agent require. But the operation of the priest in conferring the sacrament itself is mightier than in those things that are preparatory to the sacrament. And so the highest ministers who are called deacons co-operate with the priest in bestowing the sacraments themselves: for Isidore says (Epist. ad Ludifred.) that "it belongs to the deacons to assist the priests in all things that are done in Christ's sacraments, in Baptism, to wit, in the Chrism, in the Paten and Chalice"; while the inferior ministers assist the priest in those things which are preparatory to the sacraments: the readers, for instance, in catechizing; the exorcists in exorcizing.
Reply Obj. 1: The minister's operation in regard to the unclean is ministerial and, as it were, instrumental, but the priest's is principal.
Reply Obj. 2: To readers and exorcists belongs the duty of catechizing and exorcizing, not, indeed, principally, but as ministers of the priest in these things.
Reply Obj. 3: Instruction is manifold. One leads to the embracing of the Faith; and is ascribed by Dionysius to bishops (Eccl. Hier. ii) and can be undertaken by any preacher, or even by any believer. Another is that by which a man is taught the rudiments of faith, and how to comport himself in receiving the sacraments: this belongs secondarily to the ministers, primarily to the priests. A third is instruction in the mode of Christian life: and this belongs to the sponsors. A fourth is the instruction in the profound mysteries of faith, and on the perfection of Christian life: this belongs to bishops ex officio, in virtue of their office.