This is a project we worked on a while back. It is a 50x40 building with a 20 high triangular roof on top.

Inside there are little containers of water intended to be the holy water. The red carpet has pews on either side. Alter has a baptism pool and some trees for effect. One of the side rooms has a library.
Were constantly thinking of ideas to improve it and always adding new things. Ideas welcome! :)

Also, While it's not the most beautiful thing, we were quite proud of the size considering we had not yet installed the Zombe mod.

Additional Notes

Minecraft 1.7_02
(Built during 1.66)