Objection 1. It would seem that the inferior angel does not speak to the superior. For on the text (1 Corinthians 13:1), "If I speak with the tongues of men and of angels," a gloss remarks that the speech of the angels is an enlightenment whereby the superior enlightens the inferior. But the inferior never enlightens the superior, as was above explained (106, 3). Therefore neither do the inferior speak to the superior.
Objection 2. Further, as was said above (Question 106, Article 1), to enlighten means merely to acquaint one man of what is known to another; and this is to speak. Therefore to speak and to enlighten are the same; so the same conclusion follows.
Objection 3. Further, Gregory says (Moral. ii): "God speaks to the angels by the very fact that He shows to their hearts His hidden and invisible things." But this is to enlighten them. Therefore, whenever God speaks, He enlightens. In the same way every angelic speech is an enlightening. Therefore an inferior angel can in no way speak to a superior angel.
On the contrary, According to the exposition of Dionysius (Coel. Hier. vii), the inferior angels said to the superior: "Who is this King of Glory?"
I answer that, The inferior angels can speak to the superior. To make this clear, we must consider that every angelic enlightening is an angelic speech; but on the other hand, not every speech is an enlightening; because, as we have said (1), for one angel to speak to another angel means nothing else, but that by his own will he directs his mental concept in such a way, that it becomes known to the other. Now what the mind conceives may be reduced to a twofold principle; to God Himself, Who is the primal truth; and to the will of the one who understands, whereby we actually consider anything. But because truth is the light of the intellect, and God Himself is the rule of all truth; the manifestation of what is conceived by the mind, as depending on the primary truth, is both speech and enlightenment; for example, when one man says to another: "Heaven was created by God"; or, "Man is an animal." The manifestation, however, of what depends on the will of the one who understands, cannot be called an enlightenment, but is only a speech; for instance, when one says to another: "I wish to learn this; I wish to do this or that." The reason is that the created will is not a light, nor a rule of truth; but participates of light. Hence to communicate what comes from the created will is not, as such, an enlightening. For to know what you may will, or what you may understand does not belong to the perfection of my intellect; but only to know the truth in reality.
Now it is clear that the angels are called superior or inferior by comparison with this principle, God; and therefore enlightenment, which depends on the principle which is God, is conveyed only by the superior angels to the inferior. But as regards the will as the principle, he who wills is first and supreme; and therefore the manifestation of what belongs to the will, is conveyed to others by the one who wills. In that manner both the superior angels speak to the inferior, and the inferior speak to the superior.
From this clearly appear the replies to the first and second objections.
Reply to Objection 3. Every speech of God to the angels is an enlightening; because since the will of God is the rule of truth, it belongs to the perfection and enlightenment of the created mind to know even what God wills. But the same does not apply to the will of the angels, as was explained above.
Source: Summa Theologica (New Advent)