Objection 1. It would seem that the acts of the Orders are not rightly assigned in the text (Sent. iv, D, 24). Because a person is prepared by absolution to receive Christ's body. Now the preparation of the recipients of a sacrament belongs to the lower Orders. Therefore absolution from sins is unfittingly reckoned among the acts of a priest.
Objection 2. Further, man is made like to God immediately in Baptism, by receiving the character which causes this likeness. But prayer and the offering of oblations are acts directed immediately to God. Therefore every baptized person can perform these acts, and not priests alone.
Objection 3. Further, different Orders have different acts. But it belongs to the subdeacon to place the oblations on the altar, and to read the epistle; and subdeacons carry the cross before the Pope. Therefore these acts should not be assigned to the deacon.
Objection 4. Further, the same truth is contained in the Old and in the New Testament. But it belongs to the readers to read the Old Testament. Therefore it should belong to them likewise, and not to deacons, to read the New Testament.
Objection 5. Further, the apostles preached naught else but the gospel of Christ (Romans 1:15). But the teaching of the apostles is entrusted to subdeacons to be read by them. Therefore the Gospel teaching should be also.
Objection 6. Further, according to Dionysius (Eccl. Hier. v) that which belongs to a higher Order should not be applicable to a lower Order. But it is an act of subdeacons to minister with the cruets. Therefore it should not be assigned to acolytes.
Objection 7. Further, spiritual actions should rank above bodily actions. But the acolyte's act is merely corporeal. Therefore the exorcist has not the spiritual act of casting out devils, since he is of inferior rank.
Objection 8. Further, things that have most in common should be placed beside one another. Now the reading of the Old Testament must needs have most in common with the reading of the New Testament, which latter belongs to the higher ministers. Therefore the reading of the Old Testament should be reckoned the act, not of the reader, but rather of the acolyte; especially since the bodily light which the acolytes carry signifies the light of spiritual doctrine.
Objection 9. Further, in every act of a special Order, there should be some special power, which the person ordained has to the exclusion of other persons. But in opening and shutting doors the doorkeeper has no special power that other men have not. Therefore this should not be reckoned their act.
I answer that, Since the consecration conferred in the sacrament of orders is directed to the sacrament of the Eucharist, as stated above (Article 2), the principal act of each order is that whereby it is most nearly directed to the sacrament of the Eucharist. In this respect, too, one order ranks above another, in so far as one act is more nearly directed to that same sacrament. But because many things are directed to the Eucharist, as being the most exalted of the sacraments, it follows not unfittingly that one Order has many acts besides its principal act, and all the more, as it ranks higher, since a power extends to the more things, the higher it is.
Reply to Objection 1. The preparation of the recipients of a sacrament is twofold. One is remote and is effected by the ministers: another is proximate, whereby they are rendered apt at once for receiving the sacraments. This latter belongs to priests, since even in natural things matter receives from one and the same agent both the ultimate disposition to the form, and the form itself. And since a person acquires the proximate disposition to the Eucharist by being cleansed from sin, it follows that the priest is the proper minister of all those sacraments which are chiefly instituted for the cleansing of sins, namely Baptism, Penance, and Extreme Unction.
Reply to Objection 2. Acts are directed immediately to God in two ways; in one way on the part of one person only, for instance the prayers of individuals, vows, and so forth: such acts befit any baptized person. In another way on the part of the whole Church, and thus the priest alone exercises acts immediately directed to God; because to impersonate the whole Church belongs to him alone who consecrates the Eucharist, which is the sacrament of the universal Church.
Reply to Objection 3. The offerings made by the people are offered through the priest. Hence a twofold ministry is necessary with regard to offerings. One on the part of the people: and this belongs to the subdeacon who receives the offerings from the people and places them on the altar or offers them to the deacon. the other is on the part of the priest, and belongs to the deacon, who hands the offerings to the priest. This is the principal act of both Orders, and for this reason the deacon's Order is the higher. But to read the epistle does not belong to a deacon, except as the acts of lower Orders are ascribed to the higher; and in like manner to carry the cross. Moreover, this depends on the customs of Churches, because in secondary acts it is not unfitting for customs to vary.
Reply to Objection 4. Doctrine is a remote preparation for the reception of a sacrament; wherefore the announcement of doctrine is entrusted to the ministers. But the doctrine of the Old Testament is more remote than that of the New Testament, since it contains no instruction about this sacrament except in figures. Hence announcing of the New Testament is entrusted to the higher ministers, and that of the Old Testament to the lower ministers. Moreover the doctrine of the New Testament is more perfect as delivered by our Lord Himself, than as made known by His apostles. Wherefore the Gospel is committed to deacons and the Epistle to subdeacons.
This suffices for the Reply to the Fifth Objection.
Reply to Objection 6. Acolytes exercise an act over the cruet alone, and not over the contents of the cruet; whereas the subdeacon exercises an act over the contents of the cruet, because he handles the water and wine to the end that they be put into the chalice,* and again he pours the water over the hands of the priest; and the deacon, like the subdeacon, exercises an act over the chalice only, not over its contents, whereas the priest exercises an act over the contents. [The wording of St. Thomas is sufficiently vague to refer either to the Roman rite, where the priest pours the wine and water into the chalice, or to the Dominican rite, where this is done by the subdeacon.] Wherefore as the subdeacon at his ordination receives an empty chalice, while the priest receives a full chalice, so the acolyte receives an empty cruet, but the subdeacon a full one. Thus there is a certain connection among the Orders.
Reply to Objection 7. The bodily acts of the acolyte are more intimately connected with the act of Holy orders than the act of the exorcist, although the latter is, in a fashion, spiritual. For the acolytes exercise a ministry over the vessels in which the sacramental matter is contained, as regards the wine, which needs a vessel to hold it on account of its humidity. Hence of all the minor ordersthe Order of acolytes is the highest.
Reply to Objection 8. The act of the acolyte is more closely connected with the principal acts of the higher ministers, than the acts of the other minor Orders, as is self-evident; and again as regards the secondary acts whereby they prepare the people by doctrine. For the acolyte by bearing a light represents the doctrine of the New Testament in a visible manner, while the reader by his recital represents it differently, wherefore the acolyte is of higher rank. It is the same with the exorcist, for as the act of the reader is compared with the secondary act of the deacon and subdeacon, so is the act of the exorcist compared with the secondary act of the priest, namely to bind and to loose, by which man is wholly freed from the slavery of the devil. This, too, shows the degrees of Order to be most orderly. since only the three higher Orders co-operate with the priest in his principal act which is to consecrate the body of Christ, while both the higher and lower Orders co-operate with him in his secondary act, which is to loose and bind.
Reply to Objection 9. Some say that in receiving the Order the doorkeeper is given a Divine power to debar others from entering the Church, even as Christ had, when He cast out the sellers from the Temple. But this belongs to a gratuitous grace rather than to a sacramental grace. Wherefore we should reply that he receives the power to do this by virtue of his office, although others may do so, but not officially. It is the case in all the acts of the minor Orders, that they can be lawfully exercised by others, even though these have no office to that effect: just as Mass may be said in an unconsecrated building, although the consecration of a church is directed to the purpose that Mass be said there.
Source: Summa Theologica (New Advent)