5De Coelesti Hierarchia is a Pseudo-Dionysian work on angelology, written in Greek and dated to ca. the 5th century CE; it exerted great influence on scholasticism and treats at great length the hierarchies of angels. The work has also been very influential in the development of Orthodox Christian theology. Thomas Aquinas (Summa Theologica, I.108) follows the Hierarchia (6.7) in dividing the angels into three hierarchies each of which contains three orders, based on their proximity to God, corresponding to the nine orders of angels recognized by Pope St Gregory I. 1. Seraphim, Cherubim, and Thrones; 2. Dominations, Virtues, and Powers; 3. Principalities, Archangels, and Angels.