The word angel means "messenger" and this word expresses the nature of angelic service to the human race. Angels are organized into several orders, or Angelic Choirs. The most influential of these classifications was that put forward by St. Dionysius the Areopagite in the Fourth or Fifth century, in his book The Celestial Hierarchy.
In this work, the author drew on passages from the New Testament, specifically Ephesians 6:12 and Colossians 1:16, to construct a schema of three Hierarchies, Spheres or Triads of angels, with each Hierarchy containing three Orders or Choirs. In descending order of power, these were:
- First Hierarchy:
- Second Hierarchy:
- Third Hierarchy: