Unsurprisingly, they’re all on the East Coast, where European immigrants—many, of course, Catholic— first put down roots.
From Huffington Post:
Catholicism is the second largest religion in the United States at roughly 25% of the country’s population. The five most Catholic states in the U.S., according to a recent Gallup survey, are all on the East Coast: Rhode Island, New Jersey, Massachusetts, Connecticut and New York. The least Catholic states are mostly in the South.
Rhode Island’s population is 54% Catholic, more than twice the national average. As Gallup notes, the most Catholic states tend to be the least Protestant states, and vice versa.
The 24% of Americans identifying themselves as Catholic in 2013 conforms to the generally stable percentage of the population identifying themselves this way since 2008.
In addition to the high representation of Catholics on the East Coast (including the top five states plus New Hampshire), above-average proportions of Catholics are found in nearby Pennsylvania, four Midwestern states — Wisconsin, Illinois, North Dakota, and Nebraska — and two states with high Hispanic populations, New Mexico and California.
On the other hand, nine of the 10 least Catholic states are in the South, with the bottom four — Mississippi, Alabama, Tennessee, and Arkansas — all having just 8% Catholic populations. Of these 10 states, only Utah is not in the South, reflecting its predominantly Mormon population.