The hierarchic system of the angels is the result of human design, not that of the angels. Seraphim and cherubim were mentioned in the Old Testament, but then Dionysius the Areopagite, a disciple of Saint Paul, purportedly added seven more in the New Testament: thrones, dominions, virtues, powers, principalities, archangels, and angels.
Some modern religious scholars take issue with the idea of angelic hierarchies. One philosopher, Mortimer Adler, finds such speculation [of angel ranking] “highly entertaining.” And Christian evangelist writer Timothy Jones, in Celebration of Angels, states that “However tantalizing the recorded glimpses of angels in Scripture are, they are ultimately just that: glimpses. We can take great comfort, however, in knowing that populating the heavenly spheres are creatures so great they boggle and frustrate our every attempt to pin them down.”
Ranking the angels according to a hierarchy may indeed be the work of humans, but it is easy to understand why someone would create a system around which to sort all the information about angels that spans centuries, and ecclesiastical, as well as secular literature and art. A ranking system enables us to more easily assimilate the available information in order to understand the identities of these heavenly beings, their divine purposes, and importance to our lives.