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 pseudo-Dionysius the Areopagite (not to be confused with [[Dionysius the Areopagite]], who was baptized by Saint [[Apostle Paul|Paul]] and lived in the first century, and from whom pseudo-Dionysius took his name) in the fourth or fifth century in his book ''The Celestial Hierarchy''.