Click through the slideshow to see most and least Catholic states in the United States:
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Massachusetts
44,905 Catholic adherents per 100,000 people.
Rhode Island
44330 Catholic adherents per 100,000 people.
New Jersey
36,799 Catholic adherents per 100,000 people.
Connecticut
35056 Catholic adherents per 100,000 people.
New York
32443 Catholic adherents per 100,000 people.
Illinois
28439 Catholic adherents per 100,000 people.
New Mexico
28407 Catholic adherents per 100,000 people.
Pennsylvania
27578 Catholic adherents per 100,000 people.
California
27469 Catholic adherents per 100,000 people.
Louisiana
26490 Catholic adherents per 100,000 people.
Wisconsin
25,066 Catholic adherents per 100,000 people.
North Dakota
24,881 Catholic adherents per 100,000 people.
New Hampshire
23,626 Catholic adherents per 100,000 people.
Minnesota
21,689 Catholic adherents per 100,000 people.
Vermont
20,503 Catholic adherents per 100,000 people.
Nebraska
20,414 Catholic adherents per 100,000 people.
Delaware
20,328 Catholic adherents per 100,000 people.
Texas
18,586 Catholic adherents per 100,000 people.
Hawaii
18,350 Catholic adherents per 100,000 people.
South Dakota
18,286 Catholic adherents per 100,000 people.
Michigan
17,375 Catholic adherents per 100,000 people.
Ohio
17,272 Catholic adherents per 100,000 people.
Nevada
16,703 Catholic adherents per 100,000 people.
Iowa
16514 Catholic adherents per 100,000 people.
Colorado
16138 Catholic adherents per 100,000 people.
Kansas
14952 Catholic adherents per 100,000 people.
Arizona
14549 Catholic adherents per 100,000 people.
Maryland
14503 Catholic adherents per 100,000 people.
Maine
14311 Catholic adherents per 100,000 people.
Florida
13371 Catholic adherents per 100,000 people.
Montana
12898 Catholic adherents per 100,000 people.
District of Columbia
12622 Catholic adherents per 100,000 people.
Missouri
12094 Catholic adherents per 100,000 people.
Washington
11664 Catholic adherents per 100,000 people.
Indiana
11532 Catholic adherents per 100,000 people.
Wyoming
10862 Catholic adherents per 100,000 people.
Oregon
10408 Catholic adherents per 100,000 people.
Virginia
8422 Catholic adherents per 100,000 people.
Kentucky
8291 Catholic adherents per 100,000 people.
Idaho
7872 Catholic adherents per 100,000 people.
Alaska
7162 Catholic adherents per 100,000 people.
Georgia
6156 Catholic adherents per 100,000 people.
Utah
5793 Catholic adherents per 100,000 people.
West Virginia
5173 Catholic adherents per 100,000 people.
Oklahoma
4756 Catholic adherents per 100,000 people.
Arkansas
4206 Catholic adherents per 100,000 people.
Alabama
4198 Catholic adherents per 100,000 people.
North Carolina
4121 Catholic adherents per 100,000 people.
South Carolina
3929 Catholic adherents per 100,000 people.
Mississippi
3791 Catholic adherents per 100,000 people.
Tennessee
3504 Catholic adherents per 100,000 people.
A study measuring religious bodies in the United States called the, “2010 U.S. Religious Census: Religious Congregations & Membership Study (RCMS)” was recently released by the Association of Statisticians of American Religious Bodies (ASARB). The most comprehensive study of its kind, it provides detailed county-by-county information on congregations, members, adherents and attendance for 236 different faiths groups. (The survey differentiates between specific denominations within the same tradition.)
Slightly more than 58.9 million Catholic adherents and 20,589 congregations were reported in 2,960 counties across the country. The researchers found Massachusetts to be the most Catholic state with around 44.9 percent of population identifying as Catholic adherents. The researchers found Tennessee to be the least Catholic state with only 3.5 percent identifying as Catholic adherents.
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.
As the map below shows, except for a few scattered counties in the West and in the Bible belt, the rest of the country uniformly has at least 5 percent of Catholic adherents.

As far as Catholic congregations are concerned, the highest concentration is in the Northeastern and parts of the Midwest.

Interestingly, the largest Catholic churches are now in the West. In 2010, Catholic churches in the West reported an average of approximately 4,200 members. Thirty years ago, Catholic churches in the West had slightly less than 2000 members on average, meaning that churches in the West have grown by over 100 percent in the last 30 years. Catholic churches in the South have also grown about 67 percent in the same time period.

Bibliography:
Grammich, Clifford, Kirk Hadaway, Richard Houseal, Dale E. Jones, Alexei Krindatch, Richie Stanley, and Richard H. Taylor, 2012. 2010 U.S. Religion Census: Religious Congregations & Membership Study. Association of Statisticians of American Religious Bodies.
For a list of most and least religious cities, click here.
For a list of most and least interfaith cities, click here.
For a list of most and least Christian states, click here.
For a list of most and least Mormon states, click here.
For a list of most and least Jewish states, click here.
For a list of most and least Muslim states, click here.
Most and least religious states (Gallup)
The Huffington Post | By Jahnabi Barooah Posted: 07/12/2012 7:20 am Updated: 07/12/2012 7:20 am