Objection 1. It would seem that the orders of angels will not outlast the Day of Judgment. For the Apostle says (1 Corinthians 15:24), that Christ will "bring to naught all principality and power, when He shall have delivered up the kingdom to God and the Father," and this will be in the final consummaion. Therefore for the same reason all others will be abolished in that state.
Objection 2. Further, to the office of the angelic orders it belongs to cleanse, enlighten, and perfect. But after the Day of Judgment one angel will not cleanse, enlighten, or perfect another, because they will not advance any more in knowledge. Therefore the angelic orders would remain for no purpose.
Objection 3. Further, the Apostle says of the angels (Hebrews 1:14), that "they are all ministering spirits, sent to minister to them who shall receive the inheritance of salvation"; whence it appears that the angelic offices are ordered for the purpose of leading men to salvation. But all the elect are in pursuit of salvation until the Day of Judgment. Therefore the angelic offices and orders will not outlast the Day of Judgment.
On the contrary, It is written (Judges 5:20): "Stars remaining in their order and courses," which is applied to the angels. Therefore the angels will ever remain in their orders.
I answer that, In the angelic orders we may consider two things; the distinction of grades, and the execution of their offices. The distinction of grades among the angels takes place according to the difference of grace and nature, as above explained (4); and these differences will ever remain in the angels; for these differences of natures cannot be taken from them unless they themselves be corrupted. The difference of glory will also ever remain in them according to the difference of preceding merit. As to the execution of the angelic offices, it will to a certain degree remain after the Day of Judgment, and to a certain degree will cease. It will cease accordingly as their offices are directed towards leading others to their end; but it will remain, accordingly as it agrees with the attainment of the end. Thus also the various ranks of soldiers have different duties to perform in battle and in triumph.
Reply to Objection 1. The principalities and powers will come to an end in that final consummaion as regards their office of leading others to their end; because when the end is attained, it is no longer necessary to tend towards the end. This is clear from the words of the Apostle, "When He shall have delivered up the kingdom of God and the Father," i.e. when He shall have led the faithful to the enjoyment of God Himself.
Reply to Objection 2. The actions of angels over the other angels are to be considered according to a likeness to our own intellectual actions. In ourselves we find many intellectual actions which are ordered according to the order of cause and effect; as when we gradually arrive at one conclusion by many middle terms. Now it is manifest that the knowledge of a conclusion depends on all the preceding middle terms not only in the new acquisition of knowledge, but also as regards the keeping of the knowledge acquired. A proof of this is that when anyone forgets any of the preceding middle terms he can have opinion or belief about the conclusion, but not knowledge; as he is ignorant of the order of the causes. So, since the inferior angels know the types of the Divine works by the light of the superior angels, their knowledge depends on the light of the superior angels not only as regards the acquisition of knowledge, but also as regards the preserving of the knowledge possessed. So, although after the Judgment the inferior angels will not progress in the knowledge of some things, still this will not prevent their being enlightened by the superior angels.
Reply to Objection 3. Although after the Day of Judgment men will not be led any more to salvation by the ministry of the angels, still those who are already saved will be enlightened through the angelic ministry.
Source: Summa Theologica (New Advent)