The "ist" is the person who does something. If it is attached to a belief, there is a related "ism." If it is about an activity, it is usually related to a specific thing.
The world is full of "isms" and "ists." This is the 4th part of the Book of "Ists" poetically reflecting on some of the more memorable "ists" I have known.
"...isms" are often the things people believe and the "...ists" are those who believe them. The first part of a series musing on some of the ones we know best.