1738 (n.), 1768 (adj.), from episcopal + -ian. Related: Episcopalianism (by 1821).

The awkward derivative episcopalianism, seems to be used for episcopacy, a good English word, which was quite sufficient for the purposes of our honest forefathers, who were strangers to this ridiculous innovation. The word complained of is also reprehensible on the ground of its sectarian termination. ["On the Terms Episcopalian and Episcopalianism," "The Gospel Advocate," October 1821]

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Definitions of Episcopalian from WordNet

Episcopalian (adj.)
of or pertaining to or characteristic of the Episcopal church;
Synonyms: Episcopal