Satan
Patristic era

Christians have traditionally interpreted the unnamed serpent in the Garden of Eden as Satan due to Revelation 12:7, which calls Satan "that ancient serpent". This verse, however, is probably intended to identify Satan with the Leviathan, a monstrous sea-serpent whose destruction by Yahweh is prophesied in Isaiah 27:1. The first recorded individual to identify Satan with the serpent from the Garden of Eden was the second-century AD Christian apologist Justin Martyr, in chapters 45 and 79 of his Dialogue with Trypho. Other early church fathers to mention this identification include Theophilus and Tertullian. The early Christian Church, however, encountered opposition from pagans such as Celsus, who claimed in his treatise The True Word that "it is blasphemy... to say that the greatest God... has an adversary who constrains his capacity to do good" and said that Christians "impiously divide the kingdom of God, creating a rebellion in it, as if there were opposing factions within the divine, including one that is hostile to God".

The name Heylel, meaning "morning star" (or, in Latin, Lucifer), was a name for Attar, the god of the planet Venus in Canaanite mythology, who attempted to scale the walls of the heavenly city, but was vanquished by the god of the sun. The name is used in Isaiah 14:12 in metaphorical reference to the king of Babylon. Ezekiel 28:12–15 uses a description of a cherub in Eden as a polemic against Ithobaal II, the king of Tyre.

The Church Father Origen of Alexandria (c. 184 – c. 253), who was only aware of the actual text of these passages and not the original myths to which they refer, concluded in his treatise On the First Principles, which is preserved in a Latin translation by Tyrannius Rufinus, that neither of these verses could literally refer to a human being. He concluded that Isaiah 14:12 is an allegory for Satan and that Ezekiel 28:12–15 is an allusion to "a certain Angel who had received the office of governing the nation of the Tyrians", but was hurled down to Earth after he was found to be corrupt. In his apologetic treatise Contra Celsum, however, Origen interprets both Isaiah 14:12 and Ezekiel 28:12–15 as referring to Satan. According to Henry Ansgar Kelly, Origen seems to have adopted this new interpretation to refute unnamed persons who, perhaps under the influence of Zoroastrian radical dualism, believed "that Satan's original nature was Darkness." The later Church Father Jerome (c. 347 – 420), translator of the Latin Vulgate, accepted Origen's theory of Satan as a fallen angel and wrote about it in his commentary on the Book of Isaiah. In Christian tradition ever since, both Isaiah 14:12 and Ezekiel 28:12–15 have been understood as allegorically referring to Satan. For most Christians, Satan has been regarded as an angel who rebelled against God.

According to the ransom theory of atonement, which was popular among early Christian theologians, Satan gained power over humanity through Adam and Eve 's sin and Christ's death on the cross was a ransom to Satan in exchange for humanity's liberation. This theory holds that Satan was tricked by God because Christ was not only free of sin, but also the incarnate Deity, whom Satan lacked the ability to enslave. Irenaeus of Lyons described a prototypical form of the ransom theory, but Origen was the first to propose it in its fully developed form. The theory was later expanded by theologians such as Gregory of Nyssa and Rufinus of Aquileia. In the eleventh century, Anselm of Canterbury criticized the ransom theory, along with the associated Christus Victor theory, resulting in the theory's decline in western Europe. The theory has nonetheless retained some of its popularity in the Eastern Orthodox Church.

Most early Christians firmly believed that Satan and his demons had the power to possess humans, and exorcisms were widely practiced by Jews, Christians, and pagans alike. Belief in demonic possession continued through the Middle Ages into the early modern period. Exorcisms were seen as a display of God's power over Satan. The vast majority of people who thought they were possessed by the Devil did not suffer from hallucinations or other "spectacular symptoms", but "complained of anxiety, religious fears, and evil thoughts".
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