ROSEVILLE, Calif.—Residential and commercial real-estate owners aren't the only ones losing their properties to foreclosure. The past few years have seen a rapid acceleration in the number of churches losing their sanctuaries because they can't pay the mortgage.

Just as homeowners borrowed too much or built too big during boom times, many churches did the same and now are struggling as their congregations shrink and collections fall owing to rising unemployment and a weak economy.

Since...