A hierarchy of angels is a belief or tradition found in the angelology of different religions, which holds that there are different levels or ranks of angels. Higher ranks may be asserted to have greater power or authority over lower ranks, and with different ranks having differences in appearance, such as varying numbers of wings or faces. The most influential Christian angelic hierarchy was that put forward by Pseudo-Dionysius the Areopagite in the 4th or 5th century in his book De Coelesti Hierarchia (On the Celestial Hierarchy).