Objection 1. It would seem that prophetic revelation does not come through the angels. For it is written (Wisdom 7:27) that Divine wisdom "conveyeth herself into holy souls," and "maketh the friends of God, and the prophets." Now wisdom makes the friends of God immediately. Therefore it also makes the prophets immediately, and not through the medium of the angels.
Objection 2. Further, prophecy is reckoned among the gratuitous graces. But the gratuitous graces are from the Holy Ghost, according to 1 Corinthians 12:4, "There are diversities of graces, but the same Spirit." Therefore the prophetic revelation is not made by means of an angel.
Objection 3. Further, Cassiodorus [Prol. in Psalt. i] says that prophecy is a "Divine revelation": whereas if it were conveyed by the angels, it would be called an angelic revelation. Therefore prophecy is not bestowed by means of the angels.
On the contrary, Dionysius says (Coel. Hier. iv): "Our glorious fathers received Divine visions by means of the heavenly powers"; and he is speaking there of prophetic visions. Therefore prophetic revelation is conveyed by means of the angels.
I answer that, As the Apostle says (Romans 13:1), "Things that are of God are well ordered [Vulgate: 'Those that are, are ordained of God.']." Now the Divine ordering, according to Dionysius [Coel. Hier. iv; Eccl. Hier. v], is such that the lowest things are directed by middle things. Now the angels hold a middle position between God and men, in that they have a greater share in the perfection of the Divine goodness than men have. Wherefore the Divine enlightenments and revelations are conveyed from God to men by the angels. Now prophetic knowledge is bestowed by Divine enlightenment and revelation. Therefore it is evident that it is conveyed by the angels.
Reply to Objection 1. Charity which makes man a friend of God, is a perfection of the will, in which God alone can form an impression; whereas prophecy is a perfection of the intellect, in which an angel also can form an impression, as stated in I, 111, 1, wherefore the comparison fails between the two.
Reply to Objection 2. The gratuitous graces are ascribed to the Holy Ghost as their first principle: yet He works grace of this kind in men by means of the angels.
Reply to Objection 3. The work of the instrument is ascribed to the principal agent by whose power the instrument acts. And since a minister is like an instrument, prophetic revelation, which is conveyed by the ministry of the angels, is said to be Divine.
Source: Summa Theologica (New Advent)