-ist

(Greek > Latin: a suffix; one who believes in; one who is engaged in; someone who does something)

polychromatist
Someone who practices the art of using many or various colors in painting, architecture, etc.
polyculturist
Someone who cultivates a variety of crops or species.
polydemonist, polydaemonist
Someone who believes in many evil spirits.
polydiabolist
One who believes in many devils.
polygamist
Someone who commits polygamy or who has multiple marriages.
polymathist
1. Someone who is knowledgeable in a variety of subjects.
2. A person of great and diversified learning.
3. Someone of great learning in several fields of study.
polyphagist
Anyone who has an excessive appetite or who eats too much.
polyposist
A hard drinker; one who drinks excessively.
polytheist (s), (noun), polytheists (pl)
1. Someone who believes in, or worships, many gods or more than one god.
2. People who have beliefs in more than one god, usually a belief in many gods.
pomologist
popularist
positivist (s) (noun), positivists (pl)
Anyone who emphasizes facts that can be proven scientifically rather than hypothetical origins of information: A positivist maintains that sense perceptions are the only legitimate admissible basis of human judgment and precise thoughts.

Some positivists say that valid insight depends on observable facts and the sciences, beginning with mathematics and concluding with the social behaviors of humans.

posologist
Someone who specializes in the creating of and mixing or combining of medical doses.
potamologist
A specialist in the study of rivers.
potruspytophilist (s) (noun), potruspytophilists (pl)
A person who is very fond of teapots: Since Mrs. Pot loved drinking tea, she needed tea pots. With time she turned out to be a potruspytophilist who had a great collection of nice pots for brewing tea, including matching tea sets.