Objection 1. It would seem that all ecclesiastical prelates are in a state of perfection. For Jerome commenting on Titus 1:5, "Ordain... in every city," etc. says: "Formerly priest was the same as bishop," and afterwards he adds: "Just as priests know that by the custom of the Church they are subject to the one who is placed over them, so too, bishops should recognize that, by custom rather than by the very ordinance of our Lord, they are above the priests, and are together the rightful governors of the Church." Now bishops are in the state of perfection. Therefore those priests also are who have the cure of souls.
Objection 2. Further, just as bishops together with their consecration receive the cure of souls, so also do parish priests and archdeacons, of whom a gloss on Acts 6:3, "Brethren, look ye out... seven men of good reputation," says: "The apostles decided here to appoint throughout the Church seven deacons, who were to be of a higher degree, and as it were the supports of that which is nearest to the altar." Therefore it would seem that these also are in the state of perfection.
Objection 3. Further, just as bishops are bound to "lay down their life for their sheep," so too are parish priests and archdeacons. But this belongs to the perfection of charity, as stated above (2, ad 3). Therefore it would seem that parish priests and archdeacons also are in the state of perfection.
On the contrary, Dionysius says (Eccl. Hier. v): "The order of pontiffs is consummative and perfecting, that of the priests is illuminative and light-giving, that of the ministers is cleansing and discretive." Hence it is evident that perfection is ascribed to bishops only.
I answer that, In priests and deacons having cure of souls two things may be considered, namely their order and their cure. Their order is directed to some act in the Divine offices. Wherefore it has been stated above (183, 3, ad 3) that the distinction of orders is comprised under the distinction of offices. Hence by receiving a certain order a man receives the power of exercising certain sacred acts, but he is not bound on this account to things pertaining to perfection, except in so far as in the Western Church the receiving of a sacred order includes the taking of a vow of continence, which is one of the things pertaining to perfection, as we shall state further on (186, 4). Therefore it is clear that from the fact that a man receives a sacred order a man is not placed simply in the state of perfection, although inward perfection is required in order that one exercise such acts worthily.
In like manner, neither are they placed in the state of perfection on the part of the cure which they take upon themselves. For they are not bound by this very fact under the obligation of a perpetual vow to retain the cure of souls; but they can surrender it--either by entering religion, even without their bishop's permission (cf. Decret. xix, qu. 2, can. Duae sunt)--or again an archdeacon may with his bishop's permission resign his arch-deaconry or parish, and accept a simple prebend without cure, which would be nowise lawful, if he were in the state of perfection; for "no man putting his hand to the plough and looking back is fit for the kingdom of God" (Luke 9:62). On the other hand bishops, since they are in the state of perfection, cannot abandon the episcopal cure, save by the authority of the Sovereign Pontiff (to whom alone it belongs also to dispense from perpetual vows), and this for certain causes, as we shall state further on (185, 4). Wherefore it is manifest that not all prelates are in the state of perfection, but only bishops.
Reply to Objection 1. We may speak of priest and bishop in two ways. First, with regard to the name: and thus formerly bishops and priests were not distinct. For bishops are so called "because they watch over others," as Augustine observes (De Civ. Dei xix, 19); while the priests according to the Greek are "elders." [Referring to the Greek episkopos and presbyteros from which the English 'bishop' and 'priest' are derived.] Hence the Apostle employs the term "priests" in reference to both, when he says (1 Timothy 5:1)7): "Let the priests that rule well be esteemed worthy of double honor"; and again he uses the term "bishops" in the same way, wherefore addressing the priests of the Church of Ephesus he says (Acts 20:28): "Take heed to yourselves" and "to the whole flock, wherein the Holy Ghost hath placed you bishops, to rule the church of God."
But as regards the thing signified by these terms, there was always a difference between them, even at the time of the apostles. This is clear on the authority of Dionysius (Eccl. Hier. v), and of a gloss on (Luke 10:1), "After these things the Lord appointed," etc. which says: "Just as the apostles were made bishops, so the seventy-two disciples were made priests of the second order." Subsequently, however, in order to avoid schism, it became necessary to distinguish even the terms, by calling the higher ones bishops and the lower ones priests. But to assert that priests nowise differ from bishops is reckoned by Augustine among heretical doctrines (De Heres. liii), where he says that the Arians maintained that "no distinction existed between a priest and a bishop."
Reply to Objection 2. Bishops have the chief cure of the sheep of their diocese, while parish priests and archdeacons exercise an inferior ministry under the bishops. Hence a gloss on (1 Corinthians 12:2)8, "to one, helps, to another, governments [Vulgate: 'God hath set some in the church... helps, governments,' etc.]," says: "Helps, namely assistants to those who are in authority," as Titus was to the Apostle, or as archdeacons to the bishop; "governments, namely persons of lesser authority, such as priests who have to instruct the people": and Dionysius says (Eccl. Hier. v) that "just as we see the whole hierarchy culminating in Jesus, so each office culminates in its respective godlike hierarch or bishop." Also it is said (XVI, qu. i, can. Cunctis): "Priests and deacons must all take care not to do anything without their bishop's permission." Wherefore it is evident that they stand in relation to their bishop as wardens or mayors to the king; and for this reason, just as in earthly governments the king alone receives a solemn blessing, while others are appointed by simple commission, so too in the Church the episcopal cure is conferred with the solemnity of consecration, while the archdeacon or parish priest receives his cure by simple appointment; although they are consecrated by receiving orders before having a cure.
Reply to Objection 3. As parish priests and archdeacons have not the chief cure, but a certain ministry as committed to them by the bishop, so the pastoral office does not belong to them in chief, nor are they bound to lay down their life for the sheep, except in so far as they have a share in their cure. Hence we should say that they have an office pertaining to perfection rather than that they attain the state of perfection.
Source: Summa Theologica (New Advent)