Objection 1. It would seem that there is a degree of prophecy in the blessed. For, as stated above (Article 4), Moses saw the Divine essence, and yet he is called a prophet. Therefore in like manner the blessed can be called prophets.
Objection 2. Further, prophecy is a "divine revelation." Now divine revelations are made even to the blessed angels. Therefore even blessed angels can be prophets.
Objection 3. Further, Christ was a comprehensor from the moment of His conception; and yet He calls Himself a prophet (Matthew 13:57), when He says: "A prophet is not without honor, save in his own country." Therefore even comprehensors and the blessed can be called prophets.
Objection 4. Further, it is written of Samuel (Sirach 46:23): "He lifted up his voice from the earth in prophecy to blot out the wickedness of the nation." Therefore other saints can likewise be called prophets after they have died.
On the contrary, The prophetic word is compared (2 Peter 1:19) to a "light that shineth in a dark place." Now there is no darkness in the blessed. Therefore they cannot be called prophets.
I answer that, Prophecy denotes vision of some supernatural truth as being far remote from us. This happens in two ways. First, on the part of the knowledge itself, because, to wit, the supernatural truth is not known in itself, but in some of its effects; and this truth will be more remote if it be known by means of images of corporeal things, than if it be known in its intelligible effects; and such most of all is the prophetic vision, which is conveyed by images and likenesses of corporeal things. Secondly, vision is remote on the part of the seer, because, to wit, he has not yet attained completely to his ultimate perfection, according to 2 Corinthians 5:6, "While we are in the body, we are absent from the Lord."
Now in neither of these ways are the blessed remote; wherefore they cannot be called prophets.
Reply to Objection 1. This vision of Moses was interrupted after the manner of a passion, and was not permanent like the beatific vision, wherefore he was as yet a seer from afar. For this reason his vision did not entirely lose the character of prophecy.
Reply to Objection 2. The divine revelation is made to the angels, not as being far distant, but as already wholly united to God; wherefore their revelation has not the character of prophecy.
Reply to Objection 3. Christ was at the same time comprehensor and wayfarer [Cf. III, 9, seqq.]. Consequently the notion of prophecy is not applicable to Him as a comprehensor, but only as a wayfarer.
Reply to Objection 4. Samuel had not yet attained to the state of blessedness. Wherefore although by God's will the soul itself of Samuel foretold to Saul the issue of the war as revealed to him by God, this pertains to the nature of prophecy. It is not the same with the saints who are now in heaven. Nor does it make any difference that this is stated to have been brought about by the demons' art, because although the demons are unable to evoke the soul of a saint, or to force it to do any particular thing, this can be done by the power of God, so that when the demon is consulted, God Himself declares the truth by His messenger: even as He gave a true answer by Elias to the King's messengers who were sent to consult the god of Accaron (2 Kings 1).
It might also be replied [The Book of Ecclesiasticus was not as yet declared by the Church to be Canonical Scripture; Cf. I, 89, 8, ad 2] that it was not the soul of Samuel, but a demon impersonating him; and that the wise man calls him Samuel, and describes his prediction as prophetic, in accordance with the thoughts of Saul and the bystanders who were of this opinion.
Source: Summa Theologica (New Advent)