An advocate or supporter of the heteroousia doctrine in early Christian theology; another term for a heteroousian.
From heteroousia plus -ast (suffix for enthusiast or advocate), creating a term for those who actively championed the 'different essence' doctrine.
Adding '-ast' to make 'heteroousiast' shows how passionate people were about these debates—it wasn't passive belief, it was active championship of a controversial theological position that could get you exiled!
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