One who passionately supports a cause or concept. My sister is a true believer in social equality, so she is out at the protest today.

To convince or persuade one to believe or have faith in someone or something. I was really skeptical of his supposed breakthrough, but the demonstration he gave us made a believer of me. Their absolute dominance in the first game of the tournament made believers out of the entire city—now everyone is throwing their support behind the home team!