Objection 1. It would seem that all the angels are sent in ministry. For the Apostle says (Hebrews 1:14): "All are ministering spirits, sent to minister" [Vulg. 'Are they not all...?'].
Objection 2. Further, among the orders, the highest is that of the Seraphim, as stated above (Question 108, Article 6). But a Seraph was sent to purify the lips of the prophet (Isaiah 6:6-7). Therefore much more are the inferior orders sent.
Objection 3. Further, the Divine Persons infinitely excel all the angelic orders. But the Divine Persons are sent. Therefore much more are even the highest angels sent.
Objection 4. Further, if the superior angels are not sent to the external ministries, this can only be because the superior angels execute the Divine ministries by means of the inferior angels. But as all the angels are unequal, as stated above (Question 50, Article 4), each angel has an angel inferior to himself except the last one. Therefore only the last angel would be sent in ministry; which contradicts the words, "Thousands of thousands ministered to Him" (Daniel 7:10).
On the contrary, Gregory says (Hom. xxxiv in Evang.), quoting the statement of Dionysius (Coel. Hier. xiii), that "the higher ranks fulfil no exterior service."
I answer that, As appears from what has been said above (106, 3; 110, 1), the order of Divine Providence has so disposed not only among the angels, but also in the whole universe, that inferior things are administered by the superior. But the Divine dispensation, however, this order is sometimes departed from as regards corporeal things, for the sake of a higher order, that is, according as it is suitable for the manifestation of grace. That the man born blind was enlightened, that Lazarus was raised from the dead, was accomplished immediately by God without the action of the heavenly bodies. Moreover both good and bad angels can work some effect in these bodies independently of the heavenly bodies, by the condensation of the clouds to rain, and by producing some such effects. Nor can anyone doubt that God can immediately reveal things to men without the help of the angels, and the superior angels without the inferior. From this standpoint some have said that according to the general law the superior angels are not sent, but only the inferior; yet that sometimes, by Divine dispensation, the superior angels also are sent.
It may also be said that the Apostle wishes to prove that Christ is greater than the angels who were chosen as the messengers of the law; in order that He might show the excellence of the new over the old law. Hence there is no need to apply this to any other angels besides those who were sent to give the law.
Reply to Objection 2. According to Dionysius (Coel. Hier. xiii), the angel who was sent to purify the prophet's lips was one of the inferior order; but was called a "Seraph," that is, "kindling " in an equivocal sense, because he came to "kindle" the lips of the prophet. It may also be said that the superior angels communicate their own proper gifts whereby they are denominated, through the ministry of the inferior angels. Thus one of the Seraphim is described as purifying by fire the prophet's lips, not as if he did so immediately, but because an inferior angel did so by his power; as the Pope is said to absolve a man when he gives absolution by means of someone else.
Reply to Objection 3. The Divine Persons are not sent in ministry, but are said to be sent in an equivocal sense, as appears from what has been said (43, 1).
Reply to Objection 4. A manifold grade exists in the Divine ministries. Hence there is nothing to prevent angels though unequal from being sent immediately in ministry, in such a manner however that the superior are sent to the higher ministries, and the lower to the inferior ministries.
Source: Summa Theologica (New Advent)