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Medieval and Early Modern Christendom

As Christianity expanded throughout the Middle East, North Africa, and Europe, it came into contact with a variety of other religions, which it regarded as " pagan ". Christianity being a monotheist religion, Christian theologians believed that since there was only one God (the God of Christianity) the gods and goddesses with supernatural powers venerated by these "pagans" could not be genuine divinities but must actually be demons. However, they did not believe that "pagans" were deliberately worshipping devils, but were instead simply misguided and unaware of the "true" God.

Those Christian groups regarded as heretics by the Roman Catholic Church were treated differently, with theologians arguing that they were deliberately worshipping the Devil. This was accompanied by claims that such individuals engaged in acts of evil – incestuous sexual orgies, the murder of infants, and cannibalism – all stock accusations that had previously been leveled at Christians themselves in the Roman Empire. In Christian iconography, the Devil and demons were given the physical traits of figures from classical mythology, such as the god Pan, fauns, and satyrs.

The first recorded example of such an accusation being made within Western Christianity took place in Toulouse in 1022, when two clerics were tried for allegedly venerating a demon. Throughout the Middle Ages, this accusation would be applied to a wide range of Christian heretical groups, including the Paulicians, Bogomils, Cathars, Waldensians, and the Hussites. The Knights Templar were accused of worshipping an idol known as Baphomet, with Lucifer having appeared at their meetings in the form of a cat. As well as these Christian groups, these claims were also made about Europe's Jewish community. In the thirteenth century, there were also references made to a group of "Luciferians" led by a woman named Lucardis which hoped to see Satan rule in Heaven. References to this group continued into the fourteenth century, although historians studying the allegations concur that these Luciferians were likely a fictitious invention.

Within Christian thought, the idea developed that certain individuals could make a pact with Satan. This may have emerged after observing that pacts with gods and goddesses played a role in various pre-Christian belief systems, or that such pacts were also made as part of the Christian cult of saints. Another possibility is that it derives from a misunderstanding of Augustine of Hippo 's condemnation of augury in his On Christian Doctrine, written in the late fourth century. Here, he stated that people who consulted augurs were entering " quasi pacts " (covenants) with demons. The idea of the diabolical pact made with demons was popularized across Europe in the story of Faust, likely based in part on the real life Johann Georg Faust.

As the late medieval gave way to the early modern period, European Christendom experienced a schism between the established Roman Catholic Church and the breakaway Protestant movement. In the ensuing Reformation and Counter-Reformation (1517–1700 CE), both Catholics and Protestants accused each other of deliberately being in league with Satan. It was in this context that the terms "Satanist" and "Satanism" emerged.