I've been snooping around Cn. Michael Barlowe's Congregational Development BLOG (he's the Congregational Development officer for our Diocese) and have discovered a fascinating link called the American Religion Data Archives; this link lets you search by town, country, state, and find the church membership reported in that place. Check out this site and do your own search starting with maps/reports, then "counties" and see what they show for Marin.
I also like the Census link; I never knew all that information was out there for the asking! There's a lot on this BLOG...you might want to do some snooping of your own!