We recently came across this map, based on the 2010 census data, of the largest religious groups in each county of the U.S.

A few observations from the perspective of 2013:

"Millions of Catholics—whether Irish, Italian, German or Pole —lived vicariously through the wins and losses of Notre Dame’s football teams. For that vastly immigrant population Notre Dame football symbolized the triumphs of an ostracized people. It also reflected the ascendancy of U.S. Catholics into the nation’s mainstream."