Evangelical is a word to describe anyone who is super enthusiastic about something. You might meet an evangelical Christian or an evangelical cheerleader, or even an evangelical Christian cheerleader.
Evangelical is an adjective originally used to describe a relationship to a particular school of Protestantism. More generally, evangelical is used to describe a strict belief in the Christian gospel and an emphasis on personal conversion to Christianity. Outside of church, the adjective evangelical can describe someone who is an extremely enthusiastic or zealous about a particular cause. An evangelical supporter of something will speak passionately and try very hard to get you to convert to their cause.