According to the Talmud there was a very knowledgeable Torah scholar named Elishah ben Abuyah who turned heretical. Some scholars say he was a Gnostic, others say Sadducee, and still others see him as an Epicurean. His turnover was so unpalatable for the rabbis of the Talmud didn't call him by name but they called him "Acher," the other one.
I like to think of him as a man who used reason over faith (I'm not the only one) and so could not bring himself to fall in place within rabbinic norms. This intellectual primacy was what caused the rabbis to defame his name and create legends about his birth and reasons for heresy.
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