While high-minded theologians like Thomas Aquinas (1225–74) contemplated the nature of demons in treatises that interrogated the origins of evil for an elite and learned readership, the Cistercian preacher Caesarius of Heisterbach (ca. 1180–1240) laid out the scriptural evidence for their character in his widely read Dialogue on Miracles, a massive compendium of hundreds of pithy anecdotes designed for use in sermons. Caesarius devoted the fifth book of his Dialogue to the topic of demons and the danger that they posed to sinners. He prefaced this book with a succinct discussion of the biblical sources for Satan and his minions written with a clarity that anticipated its reuse by preachers at countless pulpits throughout medieval Europe.
It seems fitting to discuss the tempters after treating the topic of temptation. Demons are referred to by epithet as tempters, because whenever any temptation compels the mind to sin, they are identified as its authors or agents. If the devil tempted the first human being in Paradise and if he presumed to tempt Christ in the desert, which person will he dare not to tempt in this world? Accordingly, two angels have been assigned to every person: a good angel to protect them and a bad angel to test them.
Novice: There is no doubt in my mind about the holy angels and what they are because the writings of the prophets speak often concerning them. What demons truly are, how many there are, how malicious they are, and to what eternal flames they are appointed, I want you to show me from the writings of both the Old and New Testaments.
Monk: The testimonies that offer proof of these things are abundant. Concerning Lucifer, that is, the devil, so named because of the beauty of his creation and his fall, Isaiah said, “How have you fallen, Lucifer, you who rose in the morning?”[2] And after he became the devil and fell from heaven, the savior bore witness, when he said, “I saw Satan falling from heaven like lightning.” [3] Concerning the devil, Job said, “On a certain day when the sons of God came to stand before the Lord, Satan was also present among them.”[4] And in the psalm where David speaks concerning the traitor Judas, he said, “May the devil stand at his right hand.”[5] Habakkuk, too, when speaking of Christ, said, “The devil shall go forth before his feet.”[6] And in many other places, the scripture calls to mind the devil. John bears witness in the Apocalypse that he did not act alone and did not fall alone, when he says, “There was a great battle in heaven. Michael and his angels fought with the dragon, and the dragon fought and his angels. And they did not prevail and no place for them was found any more in heaven.”[7] Through his own malice, glorious Lucifer turned into a dragon, concerning whose beauty and attractiveness Ezekiel said, “You are the seal of resemblance, full of wisdom, and perfect in beauty. You were amidst the pleasures of the paradise of God. Every precious stone was your covering.”[8] It is thought that a tenth part of the angels fell. Because of their multitude, the apostle called them “powers of the air.”[9] Indeed, they filled the air when they fell. Concerning their pride, the prophet speaks to Christ in the psalm, “The pride of those who hate you ascends continually.”[10] And the Lord in the gospel says to the Jews: “You accomplish the works of your father, the devil. He was a liar from the start and the father of lies.”[11] That he is hostile to humankind, Job bears witness, when he says, “He will drink up a river and he will not marvel”—meaning the unbelievers, namely pagans, Jews, and heretics—“and he has faith that the Jordan will flow into his mouth,” meaning the baptized faithful.[12] The apostle Peter warns us as well, saying, “Be sober, brothers, and keep watch, because your adversary, the devil, roams about like a roaring lion seeking someone to devour. Resist him, strong in faith.”[13] What is said about one is understood concerning the rest because the singular number is often used for the plural. That they are doomed to eternal damnation is held from the words of the Lord, which he will speak to the wicked in judgment, “Go, cursed ones, into the eternal fire, which was prepared for the devil and his angels.”[14] And I think that the fifth book of this work is the proper place to discuss demons, because the philosophers refer to the number five as the apostate number, the reason being that when paired with any other odd number and multiplied, it always reveals itself either at the beginning or at the end. In the same way, the devil, withdrawing from the four-sided square of eternal stability, straightaway allies himself with wicked men, as though with uneven numbers. His iniquity is always revealed at the start or the end of any act or speech.