Objection 1. It would seem that the distinction of hierarchies and of orders is not from the nature of the angels. For hierarchy is "a sacred principality," and Dionysius places in its definition that it "approaches a resemblance to God, as far as may be" (Coel. Hier. iii). But sanctity and resemblance to God is in the angels by grace, and not by nature. Therefore the distinction of hierarchies and orders in the angels is by grace, and not by nature.
Objection 2. Further, the Seraphim are called "burning" or "kindling," as Dionysius says (Coel. Hier. vii). This belongs to charity which comes not from nature but from grace; for "it is poured forth in our hearts by the Holy Ghost Who is given to us" (Romans 5:5): "which is said not only of holy men, but also of the holy angels," as Augustine says (De Civ. Dei xii). Therefore the angelic orders are not from nature, but from grace.
Objection 3. Further, the ecclesiastical hierarchy is copied from the heavenly. But the orders among men are not from nature, but by the gift of grace; for it is not a natural gift for one to be a bishop, and another a priest, and another a deacon. Therefore neither in the angels are the orders from nature, but from grace only.
On the contrary, The Master says (ii, D. 9) that "an angelic order is a multitude of heavenly spirits, who are likened to each other by some gift of grace, just as they agree also in the participation of natural gifts." Therefore the distinction of orders among the angels is not only by gifts of grace, but also by gifts of nature.
I answer that, The order of government, which is the order of a multitude under authority, is derived from its end. Now the end of the angels may be considered in two ways.
First, according to the faculty of nature, so that they may know and love God by natural knowledge and love; and according to their relation to this end the orders of the angels are distinguished by natural gifts.
Secondly, the end of the angelic multitude can be taken from what is above their natural powers, which consists in the vision of the Divine Essence, and in the unchangeable fruition of His goodness; to which end they can reach only by grace; and hence as regards this end, the orders in the angels are adequately distinguished by the gifts of grace, but dispositively by natural gifts, forasmuch as to the angels are given gratuitous gifts according to the capacity of their natural gifts; which is not the case with men, as above explained (62, 6). Hence among men the orders are distinguished according to the gratuitous gifts only, and not according to natural gifts.
From the above the replies to the objections are evident.
Source: Summa Theologica (New Advent)