-ist

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

communist (s) (noun), communists (pl)
An individual who adheres to or practices the governing policies of collective control or ownership of aspects of social, economic, and political life: "As a communist, Jean's uncle was in favor of farm collectives and not of individual land owners."
computist (s) (noun), computists (pl)
1. A keeper of accounts, an accountant.
2. Someone who performs the computations entering into astronomical and other problem.
3. The procedure of calculating and determining something by mathematical or logical methods: "Computists are those who study and practice computing that includes programming, multimedia activities, gaming, and other computer activities."
conchologist
1. A collector and student of mollusc shells.
2. Someone who is a specialist in the branch of zoology that deals with the study of mollusks and shells.
3. A name given to the carrier-shell molluscs, based on their habit of attaching other shells, stones, etc., to their own shells.
conformist
Congregationalist (s) (noun), Congregationalists (pl)
A member of the Protestant denomination which has origins in the "Nonconformists" of England: The Congregationalists consider the individual congregation the basic unit of their church and they practice baptism of infants.

"Most Congregationalists in the United States belong to the United Church of Christ."

—From The Dictionary of Cultural Literacy
by E.D. Hirsch, Jr.; Joseph F. Kett, and James Trefil;
Houghton Mifflin Company; Boston; 1988; page 88.
congruist
Someone who is in agreement with others or who has harmonious relationships with other people.
contortionist (s) (noun), contortionists (pl)
1. An acrobat who is able to twist into unusual positions.

A circus acrobat who is wrapped up in himself or herself.

—Evan Esar
2. Anyone who bends his, or her, own body into unusual shapes; especially, for purposes of entertainment.

"A man or woman who leads a double life."

—Evan Esar
3. Someone who twists, or distorts, something; such as, a statement made by another person.
contrabandist (s) (noun), contrabandists (pl)
Relating to someone who deals with unlawful products: The police were looking for drugs, guns, and other illegal items in the house of Jim's neighbor.
contraversialist
controversialist
conventionalist
conversationalist
convivialist (s) (noun), convivialists (pl)
1. Someone who is friendly and agreeable; such as, a convivial atmosphere.
2. Anyone who is fond of feasting, drinking, and being with merry company; jovial.
copyist
1. Someone who is employed to make written copies of documents and manuscripts.
2. Anyone who makes copies of handwritten documents or music.
3. An unimaginative imitator or a mere imitator of others.
corruptionist (s), corruptionists (pl) (nouns)
One who defends or practices corruption, particularly in politics.