Gerbert of Aurillac
Also known as Pope Sylvester II (999-1003), Gerbert of Aurillac was a leading scholar of his day. According to Proffer, he was accused of Manicheanism. After he died, there was a rumor that he was a necromancer, since his learning was so great.
See the Catholic Encyclopedia on Pope Sylvester II, which notes that "He was held in high repute for his learning; the common people regarded his as a magician in league with the devil, and many legends grew up around his name. He is said to have introduced the use of Arabic figures into Western Europe, and to have invented the pendulum clock."
See also a Lecture on Gerbert from a History Class at the University of Kansas.