Objection 1. It would seem that one angel does not enlighten another. For the angels possess now the same beatitude which we hope to obtain. But one man will not then enlighten another, according to Jeremiah 31:34: "They shall teach no more every man his neighbor, and every man his brother." Therefore neither does an angel enlighten another now.
Objection 2. Further, light in the angels is threefold; of nature, of grace, and of glory. But an angel is enlightened in the light of nature by the Creator; in the light of grace by the Justifier; in the light of glory by the Beatifier; all of which comes from God. Therefore one angel does not enlighten another.
Objection 3. Further, light is a form in the mind. But the rational mind is "informed by God alone, without created intervention," as Augustine says (QQ. 83, qu. 51). Therefore one angel does not enlighten the mind of another.
On the contrary, Dionysius says (Coel. Hier. viii) that "the angels of the second hierarchy are cleansed, enlightened and perfected by the angels of the first hierarchy."
I answer that, One angel enlightens another. To make this clear, we must observe that intellectual light is nothing else than a manifestation of truth, according to Ephesians 5:13: "All that is made manifest is light." Hence to enlighten means nothing else but to communicate to others the manifestation of the known truth; according to the Apostle (Ephesians 3:8): "To me the least of all the saints is given this grace... to enlighten all men, that they may see what is the dispensation of the mystery which hath been hidden from eternity in God." Therefore one angel is said to enlighten another by manifesting the truth which he knows himself. Hence Dionysius says (Coel. Hier. vii): "Theologians plainly show that the orders of the heavenly beings are taught Divine science by the higher minds."
Now since two things concur in the intellectual operation, as we have said (105, 3), namely, the intellectual power, and the likeness of the thing understood; in both of these one angel can notify the known truth to another.
First, by strengthening his intellectual power; for just as the power of an imperfect body is strengthened by the neighborhood of a more perfect body --for instance, the less hot is made hotter by the presence of what is hotter; so the intellectual power of an inferior angel is strengthened by the superior angel turning to him: since in spiritual things, for one thing to turn to another, corresponds to neighborhood in corporeal things.
Secondly, one angel manifests the truth to another as regards the likeness of the thing understood. For the superior angel receives the knowledge of truth by a kind of universal conception, to receive which the inferior angel's intellect is not sufficiently powerful, for it is natural to him to receive truth in a more particular manner. Therefore the superior angel distinguishes, in a way, the truth which he conceives universally, so that it can be grasped by the inferior angel; and thus he proposes it to his knowledge. Thus it is with us that the teacher, in order to adapt himself to others, divides into many points the knowledge which he possesses in the universal. This is thus expressed by Dionysius (Coel. Hier. xv): "Every intellectual substance with provident power divides and multiplies the uniform knowledge bestowed on it by one nearer to God, so as to lead its inferiors upwards by analogy."
Reply to Objection 1. All the angels, both inferior and superior, see the Essence of God immediately, and in this respect one does not teach another. It is of this truth that the prophet speaks; wherefore he adds: "They shall teach no more every man his brother, saying: 'Know the Lord': for all shall know Me, from the least of them even to the greatest." But all the types of the Divine works, which are known in God as in their cause, God knows in Himself, because He comprehends Himself; but of others who see God, each one knows the more types, the more perfectly he sees God. Hence a superior angel knows more about the types of the Divine works than an inferior angel, and concerning these the former enlightens the latter; and as to this Dionysius says (Div. Nom. iv) that the angels "are enlightened by the types of existing things."
Reply to Objection 2. An angel does not enlighten another by giving him the light of nature, grace, or glory; but by strengthening his natural light, and by manifesting to him the truth concerning the state of nature, of grace, and of glory, as explained above.
Reply to Objection 3. The rational mind is formed immediately by God, either as the image from the exemplar, forasmuch as it is made to the image of God alone; or as the subject by the ultimate perfecting form: for the created mind is always considered to be unformed, except it adhere to the first truth; while the other kinds of enlightenment that proceed from man or angel, are, as it were, dispositions to this ultimate form.
Source: Summa Theologica (New Advent)