The researchers define adherents to be those with an affiliation to a congregation including children, members and attendees who are not members, and believe that the adherent measure is the most complete and comparable across religious groups. Congregations are defined as groups of people who meet regularly at a pre-announced time and location. More than 150.6 million Christian adherents and 344,894 congregations were reported across the country. It is noteworthy that less than 50 percent of of the nation's population identified as Christian adherents.
The study also goes to indicate the geographic distribution of Christian adherence among the states:
The study does not include any figures on atheists, agnostics or people who are adherents of no religion.
hat/tip to Joe.My.God