The Catechism of St. Thomas Aquinas
The sixth Sacrament is Holy Orders. There are seven orders: priesthood, deaconate, subdeaconate, acolyte, exorcist, lector, and porter. Tonsure (clerk-ship, clericatus) is not an order, but a formal profession of giving one’s life to the divine ministry. The episcopate is rather a dignity than an order. The matter of this Sacrament is that matter which is handed over to the candidate at the conferring of the order. Thus, priesthood is conferred by the handing over of the chalice, and so each order is conferred by the handing over of that matter which in a special way pertains to the ministry of that particular order. The form of this Sacrament is this: “Receive the power to offer sacrifice in the Church for the living and the dead.” And similarly power is conferred in the other orders. The minister of this Sacrament is the bishop who confers the orders. The effect of this Sacrament is an increase of grace for the performance of the duties of a worthy minister of Christ.
Concerning this Sacrament was the error of Arius, who taught that the priesthood could not be distinguished from the episcopate.
33. “That the number of ministers was wisely established, is proved by considering the various offices that are necessary for the celebration of the Holy Sacrifice of the Mass and the administration of the Blessed Sacrament. This is the chief scope of their institution. They are divided into major or sacred orders (priesthood, deaconship. subdeaconship) and minor orders. . . The bishops are placed over the various dioceses to govern, not only the other ministers of the Church, but also the faithful, and to promote their salvation with supreme care and diligence.” Roman Catechism,” “Holy Order,” 26). It is the common opinion to-day that the episcopate is an order.