'theistically' originates from Modern English, specifically from the adjective 'theistic', where the Greek root 'theos' meant 'god'.

'theistic' developed from 'theism' (early 17th century), which came via New Latin/Modern English from Greek 'theos' meaning 'god'; the adverbial suffix '-ally' (from Old French/Latin-derived '-al' + English '-ly') produced 'theistically'.

Initially it related to belief in the existence of God or gods ('theism'); over time the adverb came to mean 'in a way that reflects or expresses that belief,' which is its current use.