One aspect of Gallup's State of the States study is the importance of religion. Over 65% of all Americans say religion is an important part of their daily lives. As far as nations go, that's a pretty big number.
"At least half of the residents of all but four states (Vermont, New Hampshire, Maine and Massachusetts) say religion is important in their daily lives."
Forty-six states--including some notably liberal states--show that more than half of their residents affirm the importance of religion in their life. Granted, the study says "religion"--not a relationship with Jesus--so this doesn't necessarily connote Christian followership, but it is a reasonable assumption that Christianity comprises far and away the largest component.
In a study several years ago, researchers at Notre Dame discovered that there are actually two Bible Belts. As best as I can recall, one of the belts runs from Lubbock, Texas to Atlanta, Georgia (along I-20), continuing north on I-95 to about Richmond, Virginia. The second belt begins in Waco, Texas and runs north to Minneapolis-St. Paul--with an eastward bend to a pointwest of Chicago (around Wheaton, Illinois). Oddly enough, the two belts cross in Dallas-Ft. Worth.
At Advocace, we use some other studies that help us align marketing budgets to the Designtated Market Areas that have the greatest number of Bible and Devotional Reading Households. Our process focuses on 'people' not 'percentages' so the markets line up a bit different.
Regardless of method, the findings are similar. Some markets just have more Christian influence than others. And that culturally ingrained influence can give a Christian formatted radio station a significant boost in acceptance and funding from donors and businesses.