Hi Eddie:
Welcome to the Village from snowy, cold Minnesota.
Your glueing question depends on the material used. If gluing natural wood to plywood, I like to use some mechanical fasteners to compensate for expansion of the natural wood vs plywood. In that case, I mask the borders of the ply and apply just enough Titebond to make the bond without squeeze-out. I then use 2 mechanical fasteners on either end of the natural wood located in the middle of the piece as "insurance: against glue-joint failure.
In the case of your cross, even if your backer board is plywood, the fretwork will permit movement of your cross. I would do the gluing before finishing in that particular piece.