I think you're wrong. Unless you want all books to be called 'How to be really good at...' or equivalent, then people have to try to be original to some degree which includes using words in new contexts.

I looked up ninja (http://zillyninja.blogspot.ie/2011/02/origin-of-word-ninja.h...) and apparently it can mean "a person skilled in stealth" or "one who endures." while the ninja themselves were spies for the samurai but haven't been around for a long time (http://web-japan.org/kidsweb/explore/history/q4.html).

I guess that you'd have no problem if someone called their book 'how to become a javascript expert'. Expert comes from the latin word expertus ( past participle of experīrī to try, experience) (http://dictionary.reference.com/browse/expert?s=t) and presumably someone had to import it to english and later on someone else used it in a new unorthodox way.

BTW I looked it up and the word evangelist comes from greek and means 'bearer of good tidings', so it isn't something that has to be tied solely to religion