The phrase "great evangelist" refers to someone who is highly skilled at promoting and spreading the beliefs of a particular cause, especially a religious one. Full definition
And with bedtime always approaching, you may find one Saturday a story or message you need to pass along (listeners of the show make great evangelists). (religion-online.org)
The greatest of them was Paul, radically converted from a life of persecuting the Christians to be the first great evangelist, missionary, theologian, and administrator of the Christian Church. (religion-online.org)