When someone is known for being wildly devoted to a cause or idea, they're called a firebrand. A firebrand enjoys pushing buttons and stirring up passions.
Firebrand isn't such a tough word to remember if you think of that person's "brand" being "fiery." Someone who enjoys heating up the debate around a subject or lighting a fire under other people is a firebrand. "Fire" is their "brand." Firebrands come in all shapes and sizes: conservative, liberal, militant, creative. Anyone who takes a strong, provocative stance and challenges people with heated rhetoric might be labeled a firebrand. All it takes is guts and a willingness to stir things up.