Angels are sent by God in ministry; for that which begins to be where it was not, or by a mode in which it was not, is said to be sent. Applied to the Holy Ghost and to the Son, the term is used, not as though They began to be where They were not, but inasmuch as They exist after a new mode, by grace and the Incarnation: for a universal agent reaches everywhere, while a particular agent is so in one place as not to be in another. Thus an angel, about to do something in regard of corporeal creatures, applies its power anew to the object, and so begins to be where it was not; and since the action proceeds from God, and is ordered to something else, it is called ministry.
According to the ordinary course, the inferior angels only are sent, although superior ones may be sometimes sent by Divine dispensation. As regards the Beatific Vision, all are present before God, for all see God immediately; but only the angels of the first Hierarchy understand the mysteries contained in the Divine Essence, for the inferior are informed by the superior. In this sense, therefore, all are not said to assist, but the superior only, who are directly illuminated by God.
Nor are all the angels of the second Hierarchy said to be sent; for to be sent is to exercise some ministry towards corporeal creatures at the command of God. Hence only the executive angels are sent, while the administrative and other supreme Orders have other offices, as indicated by their names. Thus it appears that only five of the angelic Orders are sent in ministry.
- text taken from Compendium of the Summa Theologica of Saint Thomas Aquinas, by Bishop Berardus Bongiovanni