Does anyone in your church have one of those double paint rollers? I think that they are called "magic walls". We did the wall outside of our "town" library by taping (1/2" scotch tape) off our walls into big squares off set every other row to look like blocks on a wall. Then we took the two colors of latex paint that we wanted and used this double roller. The walls look like they are marble. You can take a feather and dip it in a lighter color paint and put "veins" in the marble. We are hoping to get pictures on this site. Good luck.
Ellen