There were many ancient monotheistic religions such as Judaism, but again these were tribal religions, which had no urge to prosyletise, to "spread the word". Jehova was the god of the Jews, and they felt no need to persuade other tribes to worship him as well.
Christianity changed all that. It felt an overpowering need to make their one god the god of everybody else within range, and so they invented the missionary. But to do this it was necessary to discredit the old pagan gods, and in particular the goddess and the horned god.
Pagan pantheons never include gods of evil. Pagan gods are aspects of Nature, and in Nature there can be no evil; it exists only in the human imagination. They frequently include trickster gods, such as Loki, and gods can have a reputation for being unwise to be involved with, but not actual evil. Monotheistic religions, on the other hand, require a god of evil as an adversary of their god of good. Otherwise, why would you need Him ? An obvious way to discredit the horned god and justify punishment of his worshippers is to say that he is, in fact, the god of evil in the new religion. And has been all along.
This did not happen all at once. For centuries Christianity existed alongside much older pagan beliefs all across Europe, and adapted to the people there and accomodated their beliefs as far as possible. The Celtic Christian Church in dark age Ireland is a typical example.
Being an organisation run entirely by men it took an instant dislike to the old wise women living around every country village, passing on ancient pagan folk-lore to the new generation and selling folk medicine and magic charms. The priests declared that these women were worshippers of the devil, and must be burned. During the process of extracting imaginative confessions from their bewildered captives a brand new folk myth was gradually born, as the inquisitors continually outdid one another in imagining new horrors of depravity in the worship of Lucifer, and under torture their prisoners confessed to it all. Christianity created its own reflection, and called it Satanism.
In order to improve the consistency of the confessions, and thereby create the evidence for this imaginary foul creed, a special manual was required, detailing the exact acts of which the "witches" stood accused, and would therefore confess to. It also described in excruciating detail exactly how to extract such confessions. This "Witch Hunting: How To Do It" guide was published in 1486, written by two monks, Heinrich Kramer and James Sprenger. It was called The Malleus Maleficarum, or "Hammer of the Witches".
This was the origin of Satanism as most people understand it.
Before this there had been only the Luciferians (a real heresy), who practised Christianity in most important respects but with one exception: having read the fine print in the early Coptic texts from which the common Christian bible is derived, they had noticed that the actual creation of this world was credited to Lucifer, an angel working for Jehova. So they offered up their prayers to Lucifer as "the real master of this world". They did not sacrifice babies, call up demons or do anything else distasteful. They just saw a different name on the title deeds.
In 1719 the Duke of Wharton (also the masonic Grand Lodge Grand Master) created a secret party club where the rich and powerful could get their rocks off with sex'n'drugs'n'rock'n'roll. Seventeen years later in 1746 this club was taken over by Sir Francis Dashwood who modestly renamed it The Order of Saint Francis. It was far better known as The Hellfire Club.
To give their orgies more appeal Sir Francis decided to make them evil, by enacting the satanic practises described in detail in The Malleus Malleficarum. For the first time, the actual practise of Satanism had come to exist.
Early members of this exclusive sex-club included the artist William Hogarth and parliamentary rabble-rouser John Wilkes. In 1762, the British people were surprised to learn that their Prime Minister, The Marquis of Bute, along with The Chancellor of the Exchequer, were members of this Satanic organisation. Scientist and revolutionary Benjamin Franklin joined in 1772.
Satanism had never existed before
The Malleus Maleficarum, but now anyone with an urge to kick against the
heels of the ultimate father figure had the definitive guide to work from.
Satanism was created by two devout Christian monks.
There is very little evidence that Satanism is actually practised today anywhere outside of Hammer Horror movies and heavy metal stage acts, but people being what they are it must go on in some dark corners. If so, then its enactors are performing deeds first imagined by the inquisitors of The Holy Office of the Vatican.
It is apparent that horny forest spirits
were a part of countryside folklore as late as the 18th century, and only
a hundred years after the heyday of The Inquisition Shakespeare was writing
about them in his plays. These Pan-like creatures, known as Pucks or Robin
Goodfellow, had the appearance of satyrs and the habit of copulating with
human women. In an early confession these harmless creatures must have
been mentioned and the inquisitors seized upon it, and then used it again
and again. Until now the villagers worshipping in church on Sunday and
practising simple folk magic the rest of the week had thought nothing of
it, but now Robin Goodfellow was given a new name: The Devil, and
became irredeemably evil.
Before this the devil had no clear shape: after the fifteenth century he had cloven hooves, goat legs, a tail, horns and a sex-drive. Wings were added because he was supposed to be a fallen angel. None of this is to be found anywhere in the scriptures.
Christians frequently accuse pagans of being devil worshippers, and we find this annoying. The devil is a Christian god, a part of the Christian pantheon along with The Father, The Son, The Holy Ghost, The Virgin Mary, the angels and the saints. Satanism is a Christian heresy, and we are pagans, not Christians.
