Traditionally used for formal letters, often with philosophical or moral content, not casual communication. His epistle to the community discussed ethical living.
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Literary Flair
Using 'epistle' adds a literary or sophisticated tone to a piece of writing. She styled her farewell note as an epistle.
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Rare in Speech
It's uncommon in everyday language, may sound outdated or pretentious in spoken conversation. He jokingly called his lengthy text an 'epistle'.