The film was originally completed in late 1988 and was one of the last films completed by distributor New World Pictures when they filed for Chapter 11 bankruptcy. The film's trailer was actually shown before early 1989 theatrical releases. Instead of going direct to video, Warlock was picked up and released in theatres by Trimark Pictures in 1991. Its box office success lead to a sequel, "Warlock: The Armageddon" in which Trimark also released theatrically.
The trivia items below may give away important plot points.
The scene in the theatrical trailer involving the Warlock as the satanic messiah was cut after negative reactions from test screenings. The channeler's death was originally much more violent. The scene involved a topless Mary Woronov being frozen after the devil's eyeballs emerged from her breasts. The Warlock then stomps her to pieces to free them. After the wrap up, the scene was redone as what remains in the film.
The warlock's goal is to bring together all three parts of a book that will allow him to learn the "true" name of God, and undo all of Creation by speaking the name backward. In many spiritualistic pagan religions, evil spirits were said to have personal names kept from all others, for learning the true name of a spirit would allow an entity to gain control over it.