A person who worships the devil, practices diabolism, or is devoted to extremely evil pursuits.
From diabolism + -ist (a suffix indicating a person who practices or believes in something), following patterns like 'atheist,' 'pianist,' or 'activist.'
The -ist suffix creates professional or ideological identities—it's why someone who practices Buddhism is a Buddhist, making -ist one of English's most powerful identity-creating morphemes.
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