(from: http://en.wikipedia.org/wiki/Heresy_in_Judaism#Talmudic_definition_of_heresy)
The Mishnah (Sanh. x. 1) says the following have no share in the world to come: "He who denies that the Torah is divinely revealed [lit. "comes from Heaven"], and the apiḳoros." R. Akiba says, "also he who reads heretical books" ("sefarim ḥiẓonim"). This is explained in the Talmud (Sanh. 100b) to mean "sifre Ẓeduḳim" (Sadducean writings); but this is an alteration by the censor of "sifre ha-Minim" (books of the Gnostics or Heretics). The Biblical version, "That ye seek not after your own heart" (Num. xv. 39), is explained (Sifre, Num. 115; Ber. 12b) as "Ye shall not turn to heretic views ["minut"] which lead your heart away from God" (see Maimonides, Yad, Akkum, ii. 3).