I think the 1910 figure might have more to do with practical concerns. When you have no car and live on a farm far from anything, as a huge number of people did back then, you can't regularly get to church even if you wanted to. So I wouldn't interpret that number as a clean measure of religiosity.
Go for a drive in new england sometime and count the churches in the sticks. There's usually 2-5 congregations per tiny little hamlet, most of them built in the mid-1800s or earlier; most of them now defunct or dying. Sometimes you get 3 schisms of congregationalists in a row!
"build a church and school within reasonable walking distance" was rule 1 of new towns for a good long while.
