A Bitcoin evangelist is an informal name for someone who actively promotes Bitcoin, through talks, writing, podcasts or social media, and tries to persuade people, businesses or governments to use it. An evangelist need not reject other cryptocurrencies; someone who believes Bitcoin is the only one worth having is called a Bitcoin maximalist, which is described in full on its own page.
Early evangelists were among the first to spread the ideas in the Bitcoin whitepaper published by Satoshi Nakamoto.