The Glenmary Research Center has produced “county-level choropleth maps” of America, marking religious identification. These are fascinating to review.
The map of all religious adherents indicates that the east and west coasts are more secularized than the heartland, with the Pacific Northwest the most un-churched of all major regions. The map of leading church bodies makes it appear that Baptists and Roman Catholics are dominant across the country, with the exception of Lutherans in the Midwest and Mormons in the Mountain West.
As for major Christian denominations, the Baptists are clustered in the South and Southeast, while the Methodists shift a bit further north and west. Presbyterians are more evenly distributed, while the Lutherans are perhaps the most regionally concentrated.
The Episcopalians (smaller in number) show unusual concentrations in the upper Midwest and in Alaska, while the Eastern Orthodox are clearly concentrated where immigrations patterns would suggest. The immigration pattern is at least a partial explanation for the shape of the map of Roman Catholicism as well.
No surprises when it comes to where the Unitarians are to be found in largest numbers (a smallish largest number, to be sure), and the same is also true of the United Church of Christ (with the concentrations in the Midwest explained by the old Evangelical and Reformed root of the denomination).