The suffixes -ist, -ite, and -ian all mean a follower of a person or idea. For example, a follower of Christianity is a Christian, a follower of Buddhism is a Buddhist, and a follower of Shia Islam is a Shiite. What is the difference between all of these suffixes?
From Chambers Dictionary:
“-ite” is a suffix used to “form names of people, indicating their origin, place of origin, affiliations, loyalties” (e.g. Jacobite). Whereas suffixes “-an” or “–ian” denotes “things belong to or typical of a specific person” (e.g. Johnsonian).
I couldn't find anything on -ist. They generally all mean the same thing though and I assume the reason we have all three in English is so there is always a good aesthetic/phonetic fit for various situations.