There is some evidence that “Satan worship” was an actual thing in the Middle Ages and not just something cooked up by paranoid Christians. However, you have to remember that it wasn’t full blown worship where there was a complete rejection of Christ for the devil. It was more often an acknowledgment of the devil’s power and that one could seek him out to obtain your wishes. One of the most popular cases has to be that of Gilles De Rais, who was a knightly companion of Joan of Arc and later a convicted child murderer. There is evidence that he tried to invoke the devil, with the help of an Italian alchemist named Francois Prelati, after he was facing financial difficulties. Of course, the strange thing about all of this was that Gilles De Rais was a very devout Christian. Georges Bataille gives a darkly amusing account of Gilles’ character:

Even though [Gilles De Rais] invoked the demon and expected from him the reestablishment of his fortune, up to the end he was naively a good and devout Christian. A few months before his death, still free, he confessed and approached the Sacred Altar. He even had a feeling of humility on this occasion; in the church at Machecoul, the common people moved aside, leaving room for the great lord. Gilles refused, asking the poor folk to stay beside him. This was a moment when anguish perhaps took him by the throat, when he wanted to renounce his orgies of blood. He decided then to go abroad, to go crying in front of the Holy Sepulcher in Jerusalem…. Gilles de Rais’ contradictions ultimately summarize the Christian situation, and we should not be astonished at the comedy of being devoted to the Devil, wanting to cut the throats of as many children as he could, yet expecting the salvation of his eternal soul …

Gilles De Rais even set limits on what he willing to offer the devil for the restoration of his wealth. In the pact he offered to the devil, he naturally exempted his soul and life.

What is more, we know from Prelati’s testimony that before this invocation, Gilles had given him this letter (or note) for the devil written in French in his own hand, of which this is the text: “Come at my bidding, and I will give you whatever you want, except my soul and the curtailment of my life.” The devil did not come, and François, that same day, returns the note to Gilles.

In the records Francois Prelati comes off as a typical charlatan that has managed to fool Gilles, so whether Francocis Prelati genuinely worshiped the devil himself is up for debate. In Prelati’s own confession, he admits that the demon he invoked only appeared when Gilles was absent. In another revealing incident, Prelati acted as if he was being beaten up by the demon he was invoking

His [Prelari] horror of the devil is then so great that Gilles is terrified. Gilles does not dare enter the room and weakly asks Blanchet [Gilles’ priest] to do it in his stead. The latter trembles no less than his master. Blanchet at last has the courage to look through a kind of interior window rather high up, from which one can peer into the room. Prelati does not respond to the calls; he is content with multiplying his groans. When he finally exits the room, the pitiful man, he recounts how the devil beat him horribly. Prelati thinks that the punishment he has received is due to the anger of the spirits indignant with him for having, in his conversations, held them for demons of little consequence and little power….and he had heard it said, so he says anyway, that the said demonic spirits were “begotten from material nobler than the Blessed Virgin Mary.”

Francois Prelati later even suggested that a child be sacrificed to the devil, which seems to have troubled the still devoutly Christian Gilles, so Prelati himself had to actually offer the child to the devil.

…Prelati, divining the need to take his master in hand, proposes what could be a last resort: the irritated demon asked Gilles for a sacrifice! It was time to sacrifice an infant to the Devil. At first this proposition seems to have left Gilles in anguish. Prelati must have known in advance that this superstitious man would tremble; he knew the reticence of the criminal who never ultimately abandoned the hope and anxiety to save his soul; Gilles could not dissemble what was impardonable and repugnant in the sacrifice of an innocent, of a miserable child to the “unclean spirit.” However, at bay, at all costs wanting to save, as with his soul and life, what was left of his riches, he appeared one evening carrying the hand, heart, and eye perhaps, of a child. During the night, the Italian presented the horrible offering, but the devil did not come.

Giles De Rais also stated in his confession that he himself had received a book from a certain knight in Anjou, who was imprisoned for heresy, about the evocation of the devils, which “he had returned to the said knight, not having kept it for long....”

Source:

The trials of Gilles De Rais by Georges Bataille

Gilles de Rais Jean Benedetti