Interesting word, harbinger.
Not a word we use very often. 

This is a word that could be used interchangeably, like the word announcer: 
One who makes ready for something to come. 
That is the role of every believer - not to be the way, but to pave the way for something or someone greater. 

An announcer is not the star; they prepare people for the event. 
That's the complete role and responsibility of a harbinger. 
Make way, get ready...
 He is coming!