I didn't get to know a practicing Catholic until college; it's difficult to get across to my friends how not-Catholic the area I grew up was. As of 2000, the county I grew up in had one Catholic church; the county next door, population 85,000, had three. That's four over about 800 square miles. Where I live in Chicago, I'm in walking distance of about twice that. So it never ceases to amaze me how Catholic the city is.
After every census, the Association of Religion Data Archives publishes its own U.S. Religion Census, and provides an breakdown of the nation's religious adherences. The takeaway: Cook County's a bit less Catholic than it used to be, but it's still very Catholic:
* 1971: 2,409,465
* 1980: 2,303,743
* 1990: 2,121,152
* 2000: 2,146,961
* 2010: 1,947,223
(In 1952 there were fewer than two million Catholics in Cook County, but the population was more than 500,000 fewer; unfortunately, the ARDA doesn't have data for the 1960s, perhaps because God was famously dead that decade). Of course, Cook County's population also declined from 1980 to 2010, from 5,253,655 to 5,194,675. So another way of looking at it is the "adherence rate," or Catholics per thousand:
* 1971: 438.7
* 1980: 438.5
* 1990: 415.5
* 2000: 399.3
* 2010: 374.8
By contrast, according to the non-religion-census 1951 statistics, the Catholics-per-thousand was 386.7.
Overall, the county's shed 356,520 Catholics over the past 30 years, though most of that can probably be attributed to suburbanization: Lake County added 80,000 Catholics over that period, a 60 percent gain; DuPage, 133,000, a 55 percent gain; Will County, 101,000, a 91 percent gain; Kane County, 84,000, a 118 percent gain; and so forth. Snowbirds probably account for a bit, too: the Catholic population is moving West and South, measured by regional percentages.
My own home county of Botetourt went from 664 Catholics to 807 over the course of the decade, clearly part of that larger shift, though there still may be more Catholics on my block in Chicago.
Nonetheless, Cook still has more than twice as many Catholics as evangelical, mainline, and black Protestants combined (783,984, although it's likely black Protestant denominations were undercounted). In the Chicago-Joliet-Naperville MSA, there are a mere twice as many Catholics as Protestants (1,605,000 to 3,237,000).
The big gainers? Muslims. At an estimated 201,152 in Cook County, they're just a hair behind mainline Protestants, at 202,114, with 106,000 added in just the past decade. (Only the loose Pentecostal affiliation of the Assemblies of God also added more than 10,000 adherents since 2000.) They've even outpaced Mormons as the fastest-growing religion in Illinois.
Photograph: juggernautco (CC by 2.0)