Christian angelology  In Christianity, angels are agents of God, based on angels in Judaism. 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"). During the Middle Ages, many schemes were proposed about the hierarchy of angels, some drawing on and expanding on Pseudo-Dionysius, others suggesting completely different classifications. According to medieval Christian theologians, the angels are organized into several orders, or "Angelic Choirs". Pseudo-Dionysius ("On the Celestial Hierarchy") and Thomas Aquinas ("Summa Theologica") drew on passages from the New Testament, specifically in Galatians 3:26-28, Matthew 22:24-33, Ephesians 1:21-23, and Colossians 1:16, to develop a schema of three Hierarchies, Spheres or Triads of angels, with each Hierarchy containing three Orders or Choirs. Although both authors drew on the New Testament, the Biblical canon is unclear on the subject, and these hierarchies are considered less definitive than biblical material. As referred to in the theological doctrine of the communion of saints, in Paradise there is a common and unique vision of the truth and contemplation of the Face of God, without any kind of difference between angels or human souls. The "Summa theologiae" states that there exist different degree in respect of the creation, about the power of intercession to God and of direct entrustment in the human lives. The first sphere angels serve as the heavenly servants of God the Son incarnated. Seraphim (singular "Seraph") are, literally translated, "burning ones". The word seraph is normally a synonym for serpents when used in the Hebrew Bible. Mentioned in , Seraphim are the highest angelic class and they serve as the caretakers of God's throne and continuously shout praises: "Holy, holy, holy is the Lord of hosts; the whole earth is full of His glory!" According to , the Seraphim are described as fiery six-winged beings; with two wings that cover their faces, with another two that cover their feet, and the last two they use to fly. Cherubim have four faces: one of a man, an ox, a lion, and an eagle (later adopted as the symbols of the four evangelists).