Counties with the highest population penetration of some major/notable religious groups:
Baha'i: 5.5% of Marion County, SC (and 10% of total adherents... huh?)
Catholic Church: 100% of Rolette County, ND
Islam: 29% of Emporia City, VA (?)
Mormons: 100.8% of Madison County, ID (lol)
Coptic Orthodox Church: 13.3% of Fairfax City, VA
Conservative Judaism: 7.4% of Fairfax City, VA
Orthodox Judaism: 19.3% of Rockland County, NY
Reform Judaism: 32.6% of Falls Church City, VA
Seventh Day Adventists: 11.4% of Walla Walla County, WA
Southern Baptist: 343.7% of King County, TX (lolol)
Also, some other observations:
- There are 1,400 Mormon congregations in Salt Lake County, UT.
- Pentecostal Fire-Baptized Holiness Church is kind of a badass name.
- My hometown apparently has the country's highest proportion of adherents to the Malankara Archdiocese of the Syrian Orthodox Church in North America, which I didn't even know existed. EDIT: and, googling the church's address, StreetView only shows a vacant lot- that I'm pretty sure was still vacant last time I was there. Will have to swing by there next time I'm in the area to see if there's a church there or what.