'Duck Dynasty' Stars Preaching in Church -- We're Hunting for Salvati

Here's something you probably didn't know: the stars of "Duck Dynasty" are hardcore Christian evangelists who frequently preach at their local church ... and TMZ has video of some of last weekend's sermons.

A ministry assistant at White's Ferry Road Church of Christ in West Monroe, LA tells TMZ, Phil Robertson and his family have been active at the church for more than 30 years.

We're told Phil regularly gives fiery sermons -- but last weekend, the entire family joined in, when the church celebrated Duck Commander Sunday ... a yearly tradition where the entire Robertson family preaches to the congregation in full camo (not kidding).

Funny... we're told the Duck Commander Sunday tradition existed long before the A&E reality show.

You gotta listen to the video ... these guys are intense.