"one indifferent to religious creeds, one 'that always make their interest the standard of their religion,'" 1704, originally dismissive, from anything on model of trinitarian, unitarian, etc.
Entries linking to anythingarian
anything n.
"a thing," indefinitely, late Old English aniþing, from any + thing. But Old English ænig þinga apparently also meant "somehow, anyhow" (glossing Latin quoquo modo).