Malaclypse the Elder has an even larger role in The Illuminatus! Trilogy than he ever did in real life, being that reality didn't get in the way of his activities in those books. In fact, he is an important shape-shifting character in the book, posing as Jean-Paul Sartre, Billy Graham, and the Devil.
According to Malaclypse the Elder himself, he is a 4th century B.C. Erisian priest (who was also prepared to perform services to Hermes, Dionysus, Heracles, Aphrodite, Athena, and Hera) who achieved illumination in the course of the at massacre at Melos in 830 YOLD. The Illuminati believe that death releases a certain form of energy. Given enough death and a person intent on capturing said energy, a person may be transformed into an immortal, immaterial being, or so the plot element goes.
Malaclypse the Elder also posed as Jesus after Jesus was crucified. In the form of Jesus, he introduced bingo, but asked Luke to not record it.