Anton LaVey spent the early 1960s running a private lecture circle in San Francisco out of a Victorian house painted black, drawing on a career that had already taken him through carnival work, organ playing and a stint photographing crime scenes for the police. On Walpurgisnacht, April 30, 1966, he shaved his head, announced the beginning of what he called the Age of Satan, and founded the Church of Satan. Three years later he wrote the book down.
The Satanic Bible is not a transcription of secret ritual. LaVey built it in four parts, mixing his own essays with borrowed material he was open about reusing, a section on ceremonial magic drawing on nineteenth century ritual magician Éliphas Lévi and a closing section lifting passages from Ragnar Redbeard's 1896 social Darwinist tract Might Is Right almost unchanged. What holds the parts together is the Nine Satanic Statements, LaVey's own compressed creed: indulgence over abstinence, vital existence over spiritual pipe dreams, and Satan as the best friend the church has ever had, because he has kept it in business for centuries.
Reviewers at the time mostly read it as showmanship, which LaVey never entirely denied. He had built a public persona around performance long before he built a religion, and the book reads that way in places, gleefully blasphemous, deliberately provocative. What it also did, whatever else it was, was give a new religious movement a text of its own on the day it needed one.