Thomas Aquinas,
Summa Theologiae — Question 108 - OF THE ANGELIC DEGREES OF HIERARCHIES AND ORDERS…
“ Whether the orders will outlast the Day of Judgment? (8) Whether men are taken up into the angelic orders?Art. 1 - Whether all the angels are of one hierarchy? Objection 1: It would seem that all the angels belong to one hierarchy. For since the angels are supreme among creatures, it is evident that they are ordered for the best. But the best ordering of a multitude is for it to be governed by one authority, as the Philosopher shows (Metaph. xii, Did. xi, 10; Polit. iii, 4) . Therefore as a hierarchy is nothing but a sacred principality, it seems that all the angels belong to one hierarchy. ”
