Evangelist (noun)

1. A person who seeks to convert others to the Christian faith, especially by public preaching.
2. A person who promotes a particular doctrine or system with great enthusiasm.

Origin:

Late middle english: from old french evangeliste or latin evangelista, from greek euangelistēs, from euangelizesthai "bring good news", from eu "good" + angelos "messenger".

Examples:

1. He was an evangelist for the power of education.
2. She was an evangelist for environmental causes.
3. The evangelist preached to a large crowd.
4. The evangelist's message was one of salvation and redemption.
5. The evangelist's speeches were marked by their fiery rhetoric.