Objection 1. It seems that not only a bishop can confer this sacrament. For Gregory (Regist. iv), writing to Bishop Januarius, says: "We hear that some were scandalized because we forbade priests to anoint with chrism those who have been baptized. Yet in doing this we followed the ancient custom of our Church: but if this trouble some so very much we permit priests, where no bishop is to be had, to anoint the baptized on the forehead with chrism." But that which is essential to the sacraments should not be changed for the purpose of avoiding scandal. Therefore it seems that it is not essential to this sacrament that it be conferred by a bishop.
Objection 2. Further, the sacrament of Baptism seems to be more efficacious than the sacrament of Confirmation: since it bestows full remission of sins, both as to guilt and as to punishment, whereas this sacrament does not. But a simple priest, in virtue of his office, can give the sacrament of Baptism: and in a case of necessity anyone, even without orders, can baptize. Therefore it is not essential to this sacrament that it be conferred by a bishop.
Objection 3. Further, the top of the head, where according to medical men the reason is situated (i.e. the "particular reason," which is called the "cogitative faculty"), is more noble than the forehead, which is the site of the imagination. But a simple priest can anoint the baptized with chrism on the top of the head. Therefore much more can he anoint them with chrism on the forehead, which belongs to this sacrament.
On the contrary, Pope Eusebius (Ep. iii ad Ep. Tusc.) says: "The sacrament of the imposition of the hand should be held in great veneration, and can be given by none but the high priests. Nor is it related or known to have been conferred in apostolic times by others than the apostles themselves; nor can it ever be either licitly or validly performed by others than those who stand in their place. And if anyone presume to do otherwise, it must be considered null and void; nor will such a thing ever be counted among the sacraments of the Church." Therefore it is essential to this sacrament, which is called "the sacrament of the imposition of the hand," that it be given by a bishop.
I answer that, In every work the final completion is reserved to the supreme act or power; thus the preparation of the matter belongs to the lower craftsmen, the higher gives the form, but the highest of all is he to whom pertains the use, which is the end of things made by art; thus also the letter which is written by the clerk, is signed by his employer. Now the faithful of Christ are a Divine work, according to 1 Corinthians 3:9: "You are God's building"; and they are also "an epistle," as it were, "written with the Spirit of God," according to 2 Corinthians 3:2-3. And this sacrament of Confirmation is, as it were, the final completion of the sacrament of Baptism; in the sense that by Baptism man is built up into a spiritual dwelling, and is written like a spiritual letter; whereas by the sacrament of Confirmation, like a house already built, he is consecrated as a temple of the Holy Ghost, and as a letter already written, is signed with the sign of the cross. Therefore the conferring of this sacrament is reserved to bishops, who possess supreme power in the Church: just as in the primitive Church, the fulness of the Holy Ghost was given by the apostles, in whose place the bishops stand (Acts 8). Hence Pope Urban I says: "All the faithful should. after Baptism, receive the Holy Ghost by the imposition of the bishop's hand, that they may become perfect Christians."
Reply to Objection 1. The Pope has the plenitude of power in the Church, in virtue of which he can commit to certain lower orders things that belong to the higher orders: thus he allows priests to confer minor orders, which belong to the episcopal power. And in virtue of this fulness of power the Pope, Blessed Gregory, allowed simple priests to confer this sacrament, so long as the scandal was ended.
Reply to Objection 2. The sacrament of Baptism is more efficacious than this sacrament as to the removal of evil, since it is a spiritual birth, that consists in change from non-being to being. But this sacrament is more efficacious for progress in good; since it is a spiritual growth from imperfect being to perfect being. And hence this sacrament is committed to a more worthy minister.
Reply to Objection 3. As Rabanus says (De Instit. Cleric. i), "the baptized is signed by the priest with chrism on the top of the head, but by the bishop on the forehead; that the former unction may symbolize the descent of the Holy Ghost on hint, in order to consecrate a dwelling to God: and that the second also may teach us that the sevenfold grace of the same Holy Ghost descends on man with all fulness of sanctity, knowledge and virtue." Hence this unction is reserved to bishops, not on account of its being applied to a more worthy part of the body, but by reason of its having a more powerful effect.
Source: Summa Theologica (New Advent)