The door inside the church in the village is opened by a main plot quest once you have cleared most of the starting section of Broken Valley.
Are you playing the demo on a PC or Xbox? A few people have run into missing or semi-transparent textures on the PC...
Have you checked for updated video drivers?
Try running the demo as administrator, and in the compatibility options disable desktop composition (in Vista/7).
A couple people had missing or corrupt textures fixed by doing a clean reinstall of the game. Maybe try uninstalling the demo, reboot, shut down all non-essential programs (especially anti-virus), make sure there is nothing left in the D2 folder (saves are stored elsewhere), then install the demo again.
Did you try changing the texture quality setting? How about a lower game resolution?
Missing Water Textures ?vista 64 transparent textures issue
