Thomas Aquinas,
Summa Theologiae — Question 108 - OF THE ANGELIC DEGREES OF HIERARCHIES AND ORDERS…
“ 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." ”
