The word Evangelist used to conjer up images of gospel preachers, trying to convert people to christianity. But now there seems to be new definition appearing in the english language. I first noticed this when google hired Vint Cerf as "Chief Internet Evangelist" now we have Guy Kawasaki's "The Art of Evangelism" and Jeremy Zawdony from Yahoo advertising for a "Developer Network Evangelist". Clearly something is changing...
The American Heritage Dictionary picks up some of this 'newer' meaning in definition 2 of the related word evangelism - "Militant zeal for a cause".
e·van·gel·ism (ĭ-văn'jə-lĭz'əm) pronunciation
1. Zealous preaching and dissemination of the gospel, as through missionary work.
2. Militant zeal for a cause.
Other dictionaries only seem to list the older "gospel preacher" meaning. http://dictionary.reference.com/search?q=evangelist
So we might think this whole new meaning for evangelist is an exciting new "Web 2.0" thing? Well it turns that that definition has been around for a while. The OED lists this 1978 quotation as an early example of the definition they call "A zealous advocate of a cause or promulgator of a doctrine."
"1978 P. HOWARD Weasel Words xxi. 90 It makes sense to say that the French Revolution found its evangelist in Rousseau; that Mary Whitehouse is the evangelist of cleaning up television. "
That being said, this meaning is a a lot newer than when it was first use to refer to a gospel writer (Mathew, Mark, Luke or John).
"1297 R. GLOUC. (1724) 67 And sende Sent Mark be euangelist in to Egypt. "
Quotations taken from the Oxford English Dictionary