So, the "-ist" suffix is used to denote a person who is related to the word or word root that the "-ist" suffix is attached to (e.g. practicing that profession, adhering to that philosophy, etc.)
However, when you want to convert it into a noun that describes the pursuit that the "-ist" suffix person is engaged in, sometimes you use the "-ism" suffix, and sometimes you use the "-istry" suffix - a dentist is a person who engages in dentistry, a sophist is someone who engages in sophistry, a terrorist is someone who engages in terrorism, and an antagonist is someone who engages in antagonism.
Why do some of these words use the "-ism" suffix, and some use the "-istry" suffix? Is it something to do with the etymology of the word roots, is it some subtle grammatical or meaning difference, or is it just one of those inconsistent things that the English language does sometimes? I just tried looking on Google, and I can't find anything useful.