The adjective protestant describes a person or thing that is protesting or displaying disapproval or objection. You might see protestant young students marching in front of the capital in reaction to the government's newly passed bill.
Protestant originates from the Latin word protestari, meaning "declare publicly, testify, protest," which combines pro meaning "forth, before," and testari meaning "testify." A protestant person typically is someone making a public declaration against something he opposes. The protestant residents in your city are against the plan to develop businesses on the waterfront. You were being protestant when you questioned your professor's grading policy in front of the entire class.