When I was a kid there was a game show on telelvision called Let’s Make a Deal hosted by Monte Hall. The game revolved around the contestants wagering that the prize they had in their hand was not as good as one of the prizes behind door number one, two, or three. The great mystery was what was behind the door.
In the same way, the door as a motif in funerary art symbolizes mystery. The door is the pathway from the earthly realm to the next. In Christianity, however, this is almost always viewed with hope, charity, and faith. The next life will be better.