Justin Bieber Surprises Youth Gospel Group Dancing to New Song 'Holy'

Justin Bieber had the surprise of a lifetime for a bunch of beliebers dancing to his new hit song ... sneaking up on them before making the big reveal.

Justin made jaws drop Friday afternoon outside a fire station in West Hollywood, where this group of youngsters was working on a dance routine to "Holy," Justin's new collab with Chance The Rapper.

We're told Justin caught wind the local youth gospel group was gonna be rehearsing a performance based on his track ... so he headed over to the firehouse to surprise them and show support.

It's pretty cool ... you can see Justin emerge from inside the fire station as the boys and girls dance and sing with their backs to him, then he posts up on the side and watches for a while behind his face mask.

Justin finally revealed himself, taking off his mask and saying a few words ... and ya gotta see everyone's reaction.