An Evangelist is from Complete Divine (I can post up the stats if you need them).
Basically, it's a divine style bard. I think you get caster level progression (I'd have to check, more tomorrow) and bardic music progression.
In addition, you get abilities such as inspire hope (bonus to saves vs. fear), inflame the righteous (allies get flame shield), only half penalty for making a diplomacy check as a full round action, and at 5th level (it's a 5 level PrC) you can Convert someone (Will Save DC 10 + Evangelist Level + Cha mod), who is then treated as charmed for a long duration, and can become permanently a part of your faith.
I'm basically planning a missionary. Convert the heathens, with the diplomacy check to back it up. Wouldn't make him sing, but rather pump up Perform: Oratory and the like. I might or might not give him Vow of Peace.
Here's a quick question: Do Synergy bonuses stack (Specifically, from Bluff, Knowledge: Nobility and Royalty, and Sense Motive to Diplomacy).