Objection 1. It would seem that an angel cannot enlighten man. For man is enlightened by faith; hence Dionysius (Eccl. Hier. iii) attributes enlightenment to baptism, as "the sacrament of faith." But faith is immediately from God, according to Ephesians 2:8: "By grace you are saved through faith, and that not of yourselves, for it is the gift of God." Therefore man is not enlightened by an angel; but immediately by God.
Objection 2. Further, on the words, "God hath manifested it to them" (Romans 1:19), the gloss observes that "not only natural reason availed for the manifestation of Divine truths to men, but God also revealed them by His work," that is, by His creature. But both are immediately from God--that is, natural reason and the creature. Therefore God enlightens man immediately.
Objection 3. Further, whoever is enlightened is conscious of being enlightened. But man is not conscious of being enlightened by angels. Therefore he is not enlightened by them.
On the contrary, Dionysius says (Coel. Hier. iv) that the revelation of Divine things reaches men through the ministry of the angels. But such revelation is an enlightenment as we have stated (106, 1; 107, 2). Therefore men are enlightened by the angels.
I answer that, Since the order of Divine Providence disposes that lower things be subject to the actions of higher, as explained above (Question 109, Article 2); as the inferior angels are enlightened by the superior, so men, who are inferior to the angels, are enlightened by them.
The modes of each of these kinds of enlightenment are in one way alike and in another way unlike. For, as was shown above (Question 106, Article 1), the enlightenment which consists in making known Divine truth has two functions; namely, according as the inferior intellect is strengthened by the action of the superior intellect, and according as the intelligible species which are in the superior intellect are proposed to the inferior so as to be grasped thereby. This takes place in the angels when the superior angel divides his universal concept of the truth according to the capacity of the inferior angel, as explained above (Question 106, Article 1).
The human intellect, however, cannot grasp the universal truth itself unveiled; because its nature requires it to understand by turning to the phantasms, as above explained (84, 7). So the angels propose the intelligible truth to men under the similitudes of sensible things, according to what Dionysius says (Coel. Hier. i), that, "It is impossible for the divine ray to shine on us, otherwise than shrouded by the variety of the sacred veils." On the other hand, the human intellect as the inferior, is strengthened by the action of the angelic intellect. And in these two ways man is enlightened by an angel.
Reply to Objection 1. Two dispositions concur in the virtue of faith; first, the habit of the intellect whereby it is disposed to obey the will tending to Divine truth. For the intellect assents to the truth of faith, not as convinced by the reason, but as commanded by the will; hence Augustine says, "No one believes except willingly." In this respect faith comes from God alone.
Secondly, faith requires that what is to be believed be proposed to the believer; which is accomplished by man, according to Romans 10:17, "Faith cometh by hearing"; principally, however, by the angels, by whom Divine things are revealed to men. Hence the angels have some part in the enlightenment of faith. Moreover, men are enlightened by the angels not only concerning what is to be believed; but also as regards what is to be done.
Reply to Objection 2. Natural reason, which is immediately from God, can be strengthened by an angel, as we have said above. Again, the more the human intellect is strengthened, so much higher an intelligible truth can be elicited from the species derived from creatures. Thus man is assisted by an angel so that he may obtain from creatures a more perfect knowledge of God.
Reply to Objection 3. Intellectual operation and enlightenment can be understood in two ways.
First, on the part of the object understood; thus whoever understands or is enlightened, knows that he understands or is enlightened, because he knows that the object is made known to him.
Secondly, on the part of the principle; and thus it does not follow that whoever understands a truth, knows what the intellect is, which is the principle of the intellectual operation. In like manner not everyone who is enlightened by an angel, knows that he is enlightened by him.
Source: Summa Theologica (New Advent)