Catechism of the Council of Trent: Holy Orders
The Major Orders
Subdeacon
From the minor orders, which are not sacred, and of which we have been speaking until now, one lawfully enters and ascends to major and Sacred Orders.
Now the subdiaconate is the first degree of (major orders). Its function, as the name itself indicates, is to serve the deacon at the altar. It is the subdeacon who should prepare the altar linen, the vessels and the bread and wine necessary for the celebration of the Holy Sacrifice. He also it is who presents water to the Bishop or priest when he washes his hands during the Sacrifice of the Mass. It is also the subdeacon who now reads the Epistle which in former times was read at Mass by the deacon. He assists as witness at the Holy Sacrifice, and guards the celebrant from being disturbed by anyone during the sacred ceremonies.
The various duties that pertain to the subdeacon are indicated by the solemn ceremonies used at his ordination. In the first place the Bishop warns him that the obligation of perpetual continence is attached to this order, and declares that no one is to be admitted among the subdeacons who is not ready and willing to accept the obligation in question. Then, after the solemn recitation of the Litanies, the Bishop enumerates and explains the duties and functions of the subdeacon. Thereupon each one of those who are being ordained receives the chalice and sacred paten from the Bishop; and, to show that he is to serve the deacon, the subdeacon receives from the archdeacon cruets filled with wine and water, together with a basin and towel with which to wash and dry the hands. At the same time the Bishop pronounces these words: See what sort of ministry is entrusted to you; I admonish you therefore, to show yourself worthy to please God. Other prayers follow, and finally, when the Bishop has clothed the subdeacon with the sacred vestments, for each of which there are special words and ceremonies, he gives him the book of the Epistles, saying: Receive the book of the Epistles with power to read them in the Holy Church of God, as well for the living as for the dead.
Deacon
The second degree of Sacred Orders is that of the deacons, whose functions are much more extensive and have always been regarded as more holy. His duty it is to be always at the side of the Bishop, guard him while he preaches, serve him and the priest during the celebration of the divine mysteries, as well as during the administration of the Sacraments, and to read the Gospel in the Sacrifice of the Mass. In former times he frequently warned the faithful to be attentive to the holy mysteries; he administered our Lord's blood in those churches in which the custom existed that the faithful should receive the Eucharist under both species; and to him was entrusted the distribution of the Church's goods, as well as the duty of providing for all that was necessary to each one's sustenance. To the deacon also, as the eye of the Bishop, it belongs to see who they are in the city that lead a good and holy life, and who not; who are present at the Holy Sacrifice and sermons at appointed times, and who not; so that he may be able to give an account of all to the Bishop, and enable him to admonish and advise each one privately, or to rebuke and correct publicly, according as he may deem more profitable. He should also read out the list of the catechumens and present to the Bishop those who are to be admitted to orders. Finally in the absence of a Bishop or priest, he can explain the Gospel, but not from the pulpit, thus letting it be seen that this is not his proper office.
The Apostle shows the great care that should be taken that no one unworthy of the diaconate be promoted to this order, when in his Epistle to Timothy he sets forth a deacon's character, virtues and integrity. The same point is also gathered from the rites and solemn ceremonies which the Bishop employs when ordaining him. The Bishop uses more numerous and more solemn prayers at the ordination of a deacon than at that of a subdeacon, and he also adds other kinds of sacred vestments. Moreover, he imposes hands on him, just as we read the Apostles used to do when ordaining the first deacons. Finally, he hands him the book of the Gospels, with these words: Receive the power to read the Gospel in the Church of God both for the living and the dead in the name of the Lord.
Priest
The third and highest degree of all Sacred Orders is the priesthood. The Fathers of the first centuries usually designated those who had received this order by two names. At one time they called them presbyters—a Greek word signifying elders, not only because of the ripe years very necessary for this order, but much more on account of their gravity, knowledge and prudence; for it is written: Venerable old age is not that of long time nor counted by the number of years; but the understanding of a man is grey hairs and an unspotted life is old age. At other times they call them priests, both because they are consecrated to God, and because to them it belongs to administer the Sacraments and take charge of things sacred and divine.
Twofold Priesthood
But as Sacred Scripture describes a twofold priesthood, one internal and the other external, it will be necessary to have a distinct idea of each to enable pastors to explain the nature of the priesthood now under discussion.
The Internal Priesthood
Regarding the internal priesthood, all the faithful are said to be priests, once they have been washed in the saving waters of Baptism. Especially is this name given to the just who have the Spirit of God, and who, by the help of divine grace, have been made living members of the great High-priest, Jesus Christ; for, enlightened by faith which is inflamed by charity, they offer up spiritual sacrifices to God on the altar of their hearts. Among such sacrifices must be reckoned every good and virtuous action done for the glory of God.
Hence we read in the Apocalypse: Christ hath washed us from our sins in his own blood and hath made us a kingdom, and priests to God and his Father. In like manner was it said by the Prince of the Apostles: Be you also as living stones built up, a spiritual house a holy priesthood offering up spiritual sacrifices acceptable to God by Jesus Christ; while the Apostle exhorts us to present our bodies a living sacrifice holy , pleasing unto God your reasonable service. And long before this David had said: A sacrifice to God is an afflicted spirit: a contrite and humble heart O God thou wilt not despise. All this clearly regards the internal priesthood.
The External Priesthood
The external priesthood, on the contrary, does not pertain to the faithful at large, but only to certain men who have been ordained and consecrated to God by the lawful imposition of hands and by the solemn ceremonies of holy Church, and who are thereby devoted to a particular sacred ministry.
This distinction of the priesthood can be seen even in the Old Law. That David spoke of the internal priesthood, we have just shown. On the other hand, everyone knows the many and various precepts given by the Lord to Moses and Aaron regarding the external priesthood. Along with this He appointed the whole tribe of Levi to the ministry of the Temple, and He forbade by law that anyone belonging to another tribe should dare to intrude himself into that function. Hence it was that King Ozias was afflicted with leprosy by the Lord for having usurped the sacerdotal ministry, and had to suffer grave chastisements for his arrogance and sacrilege.
Now as the same distinction (of a twofold) priesthood may be noted in the New Law, the faithful should be cautioned that what we are now about to say concerns that external priesthood which is conferred on certain special individuals. This alone belongs to the Sacrament of Holy Orders.
Functions of the Priesthood
The office of a priest, then, is to offer Sacrifice to God and to administer the Sacraments of the Church. This is proved by the very ceremonies used at his ordination. When ordaining a priest, the Bishop first of all imposes hands on him, as do all the other priests who are present. Then he puts a stole on his shoulders and arranges it over his breast in the form of a cross, declaring thereby that the priest is clothed with power from on high, enabling him to carry the cross of Christ our Lord and the sweet yoke of God's law, and to inculcate this law not only by words, but also by the example of a most holy and virtuous life.
He next anoints his hands with holy oil, and then gives him the chalice with wine and the paten with a host, saying at the same time: Receive the power to offer Sacrifice to God and to celebrate Masses both for the living and for the dead. By these words and ceremonies the priest is constituted an interpreter and mediator between God and man, which indeed must be regarded as the principal function of the priesthood.
Lastly, placing his hands a second time on the head (of the person ordained the Bishop) says: Receive the Holy Ghost; whose sins you shall forgive they are forgiven them, and whose sins you shall retain they are retained, thus communicating to him that divine power of forgiving and retaining sin which was given by our Lord to His disciples. Such, then, are the special and principal functions of the sacerdotal order.
Degrees of the Priesthood
Priests
Now although (the sacerdotal order) is one alone, yet it has various degrees of dignity and power. The first degree is that of those who are simply called priests, and of whose functions we have hitherto been speaking.