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He would be better described as an infidel since he was never outwardly associated with Christianity (e.g., Unitarians, Mormons, JWs, Oneness, etc.).

But all those groups you named claim to be Christian. Why can't we say a heretic is someone who denies foundational principles of the Biblical Christian faith.

Now, perhaps a better discussion would be over what these foundational principles are.

True godliness is a sincere feeling which loves God as Father as much as it fears and reverences Him as Lord, embraces His righteousness, and dreads offending Him worse than death~ Calvin