noun

a person who supports an idea or theory and tries to persuade people of its truth or benefits.

an early exponent of the teachings of Thomas Aquinas

advocate,supporter,proponent,upholder,backer,defender,champion,promoter,propagandist,spokesperson,spokesman,spokeswoman,speaker,campaigner,fighter,battler,crusader,missionary,evangelist,pioneer,apostle,enthusiast,apologist,arguer,expounder