Objection 1. It would seem that there ought not to be an episcopal power above the priestly Order. For as stated in the text (Sent. iv, D, 24) "the priestly Order originated from Aaron." Now in the Old Law there was no one above Aaron. Therefore neither in the New Law ought there to be any power above that of the priests.
Objection 2. Further, powers rank according to acts. Now no sacred act can be greater than to consecrate the body of Christ, whereunto the priestly power is directed. Therefore there should not be an episcopal above the priestly power.
Objection 3. Further, the priest, in offering, represents Christ in the Church, Who offered Himself for us to the Father. Now no one is above Christ in the Church, since He is the Head of the Church. Therefore there should not be an episcopal above the priestly power.
On the contrary, A power is so much the higher according as it extends to more things. Now the priestly power, according to Dionysius (Eccl. Hier. v), extends only to cleansing and enlightening, whereas the episcopal power extends both to this and to perfecting. Therefore the episcopal should be above the priestly power.
Further, the Divine ministries should be more orderly than human ministries. Now the order of human ministries requires that in each office there should be one person to preside, just as a general is placed over soldiers. Therefore there should also be appointed over priests one who is the chief priest, and this is the bishop. Therefore the episcopal should be above the priestly power.
I answer that, A priest has two acts: one is the principal, namely to consecrate the body of Christ. the other is secondary, namely to prepare God's people for the reception of this sacrament, as stated above (37, 2,4). As regards the first act, the priest's power does not depend on a higher power save God's; but as to the second, it depends on a higher and that a human power. For every power that cannot exercise its act without certain ordinances, depends on the power that makes those ordinances. Now a priest cannot loose and bind, except we presuppose him to have the jurisdiction of authority, whereby those whom he absolves are subject to him. But he can consecrate any matter determined by Christ, nor is anything else required for the validity of the sacrament; although, on account of a certain congruousness, the act of the bishop is pre-required in the consecration of the altar, vestments, and so forth. Hence it is clear that it behooves the episcopal to be above the priestly power, as regards the priest's secondary act, but not as regards his primary act.
Reply to Objection 1. Aaron was both priest and pontiff, that is chief priest. Accordingly the priestly power originated from him, in so far as he was a priest offering sacrifices, which was lawful even to the lesser priests; but it does not originate from him as pontiff, by which power he was able to do certain things; for instance, to enter once a year the Holy of Holies, which it was unlawful for the other priests to do.
Reply to Objection 2. There is no higher power with regard to this act, but with regard to another, as stated above.
Reply to Objection 3. Just as the perfections of all natural things pre-exist in God as their exemplar, so was Christ the exemplar of all ecclesiastical offices. Wherefore each minister of the Church is, in some respect, a copy of Christ, as stated in the text (Sent. iv, D, 24). Yet he is the higher who represents Christ according to a greater perfection. Now a priest represents Christ in that He fulfilled a certain ministry by Himself, whereas a bishop represents Him in that He instituted other ministers and founded the Church. Hence it belongs to a bishop to dedicate a thing to the Divine offices, as establishing the Divine worship after the manner of Christ. For this reason also a bishop is especially called the bridegroom of the Church even as Christ is.
Source: Summa Theologica (New Advent)