Before 1830, little religion of any sort was practiced on the Illinois frontier. Energetic Protestant missionaries set out to evangelize this un-Christian population and they largely succeeded. By 1890, 36% of the adults in Illinois were affiliated with evangelical denominations—chiefly Methodist, Disciples of Christ, Baptist, Congregationalist, and Presbyterian—while 35%, mostly immigrants, belonged to liturgical denominations (chiefly Roman Catholic, Lutheran, and Episcopal). The remaining adults acknowledged no particular denomination.
Illinois has had episodes of religious bigotry: at Carthage in 1844 the Mormon founder Joseph Smith was killed by a mob, and strong but brief waves of anti-Catholicism developed in the 1850s (the "Know-Nothing" movement) and 1920s (the Ku Klux Klan). Robert Green Ingersoll, a self-proclaimed agnostic, was appointed attorney general of Illinois in 1867–69, but his identity as an agnostic prevented him from ever being elected into politics. Nevertheless, tolerance of religious diversity has been the norm for most of the state's history.
In 2000, the largest religious institution was the Roman Catholic Church, with 3,874,933 adherents in 1,225 congregations. The largest Protestant denomination was the United Methodist Church, with 365,182 adherents, followed by the Southern Baptist Convention with 305,838 adherents. Other major Protestant groups include the Evangelical Lutheran Church in America with 279,724 adherents and the Lutheran Church—Missouri Synod with 278,008 adherents. The Jewish population was estimated at 270,000 in 2000 and the Muslim community had about 125,203 adherents. There are over 11,000 Mennonites throughout the state. About 44.7% of the population were not counted as members of any religious organization.