I think that Diligent and Luke have put their fingers on it. A person isn't a heretic simply because he believes one goofy little thing, or because he doesn't believe one non-essential thing. Eating meat is a good example: I had brothers at PBI who believed that eating meat was, if not a sin, then at least displeasing to God. But they weren't heretics. If Aussiemama is a No-Gapper, that doesn't make her a heretic. If, on the other hand, she taught that the virgin birth was false, or that Jesus and Satan were "spirit brothers" (like the Mormons say), then she'd be a heretic. A heretic is somebody who perverts and twists the essential truths of God's word.

(Are Buddhists and Muslims heretics? Well, Muhammad was; he twisted the truth about Jesus. But most Muslims and Hindus and such start out from a non-Christian position, and many of them have never even heard the Gospel; they believe in false religions, but I wouldn't call them heretics. A heretic distorts and denies the Christian revelation.)

Also, there's a difference between heretics and apostates. If a saved man, like a "great evangelist," starts preaching that all men will be saved, then he's an apostate: he's "fallen away" from the truth. (That's what apostasy means: "to fall away from a standing position.") Bible correctors are apostates, but not necessarily heretics.

I would be uncomfortable calling John Calvin a heretic, although some of his doctrines were heretical. He believed the basics: the Deity of Christ, the necessity of salvation, etc. I don't think he "fell away" from the truth about baby-sprinkling or predestination, because he never knew those doctrines to begin with: he was still suffering from a "Catholic hangover," like Luther was when he hated the Jews. They were great warriors for God, in their time; but, like Solomon and Job's comforters, their doctrine was just kinda screwed up.

Specific examples: in my understanding, Rick Warren is not a heretic; he's an apostate. But Kenneth Hagin, who teaches that Jesus was "born again" in Hell, is a heretic. And anyone who teaches that speaking in tongues or water baptism are necessary for salvation are heretics, because they've tampered with the very basic doctrine of salvation. If Ruckman believed that little green men from Mars were poisoning our water supply, he'd simply be making a goofy, stupid mistake; if he taught that those little green men were divine, like Jesus, he'd be a first-class heretic.

Anyway, that's my understanding.