"_ist" is used, among other things, to denote a person with certain beliefs. I can't see that there's any element of offence (on either side) in anyone referring to myself as an atheist, or to the Archbishop of Canterbury as a deist.
"_ist" is also, of course, simply used to relate a person to certain things or activities, as in 'cyclist' or 'dentist'.
"_ism" has a variety of uses. These include forming nouns, from 'catchwords', to denote certain systems or principles (as in 'Conservatism') or forming nouns, from 'root nouns' (such as 'hero') which denote a type of conduct related to those nouns (as in 'heroism').
"_logy" is derived from the Greek for 'science (of)'. So 'psychology' is (loosely) 'the science of the mind'.
Words ending in "_logy" are nouns. The adjectives and adverbs which are derived from them end in "_logical". So a 'psychological thriller' is one which has a relationship to psychology.
Chris