'theistical' originates from Modern English, specifically formed from 'theistic' (from 'theist') with the adjectival suffix '-ical', where 'theist' ultimately comes from Greek 'theos' meaning 'god' and '-ical' meant 'pertaining to'.

'theistical' developed from earlier English forms such as 'theist' (from medieval/early modern English 'theist') and the adjective 'theistic'; the suffix '-ical' was added to form 'theistical', paralleling 'theistic'.

Initially it meant 'pertaining to a belief in God' and over time it has retained that core meaning of 'relating to or characteristic of theism'.