-ist

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

ademonist
Anyone who denies the existence of demons or the Devil.
adiabolist
A person denying existence of the Devil.
adventist
aerobiologist (s) (noun), aerobiologists (pl)
Someone who studies or specializes in the study of airborne microorganisms or spores.
aerologist (s) (noun), aerologists (pl)
1. A specialist in the branch of meteorology concerned with the study of air, especially in the upper atmosphere.
2. Someone who studies total atmospheric meteorology as opposed to surface-based applications.
aeromantist
Someone who foretells the future by examining air or atmospheric conditions; such as, a weather forecaster.
agamist
An unmarried person.
agathist
1. Someone who believes in the doctrine of agathism which is, "all things tend towards ultimate good".
2. A person who believes that the world and things in general are heading for the better; therefore, in things that most people regard as evil or tragic including virulent diseases, calamitous earthquakes, or wars, the agathist can find some ultimate purpose for good.

The optimist sees the present as pretty much for the best; the agathist, less content with the present, thinks things are inevitably tending towards good despite the bad situations which precede the ultimate results.

agonist (s) (noun), agonists (pl)
1. In physiology, a muscle whose contraction is directly responsible for the movement of a part of the body.
2. A contender for prizes in competitions.
3. A person who is engaged in a contest or struggle; such as, a protagonist in a drama or a story.
agricolist
A cultivator of the soil; an agriculturist.
agriculturalist
Someone who is engaged in the science, art, and business of cultivating soil, producing crops, and raising livestock; farming.
Whoever could make two ears of corn . . . grow upon a spot of ground where only one grew before, would deserve better of mankind . . . than the whole race of politicians put together.
—Jonathan Swift (1667-1745)
agriologist (s) (noun), agriologists (pl)
Someone who is versed in the history and customs of primitive or uncivilized peoples.
agrizoiatrist
A veterinarian who deals with wild animals.
agrobiologist (s) (noun), agrobiologists (pl)
An expert or specialist in the study of plant nutrition and growth in relation to the conditions of soils.
agroecologist (s) (noun), agroecologists (pl)
An expert in or a student of the design, development, and management of sustainable agricultural systems: "An agroecologist studies the applications of ecological principles while considering existing social, cultural, and economic factors of farming communities."