-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 (s) (noun), adiabolists (pl)
A person who denys the existence of the Devil.
adventist (s) (noun), Adventists (pl)
A member of the Seventh-day Adventist Church; a person who believes in the Second Advent: Susan was an Adventist who had faith in the imminent advent of Christ.
aerobiologist (s) (noun), aerobiologists (pl)
Someone who specializes in the study of airborne microorganisms or spores: Jack's dad was an aerobiologist who studied the effects of viruses, pollen, and pollutants that were transported by air currents.
aerologist (s) (noun), aerologists (pl)
A specialist in the branch of meteorology concerned with the study of air, especially in the upper atmosphere: Linda's father was an aerologist and did research on total atmospheric meteorology as opposed to surface-based applications.
aeromantist (s) (noun), aeromantists (pl)
An individual who foretells the future by examining air or atmospheric conditions: An aeromantist can be a weather forecaster.or a person who believes in being able to interpret a person's future by observing the characteristics in the atmosphere.
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."