Complex sentence: He taught that eating the "fruit" in the Garden of Eden was taking heed of the devil's words which led to an act of sexual intercourse between Eve and the devil-possessed serpent, producing Cain as a result of their union.
Split into simpler sentences: He taught that eating the "fruit" in the Garden of Eden was taking heed of the devil's words. This resulted in a sexual union between Eve and the devil-possessed serpent, which produced Cain as a result of their union.