Connecticut's religious development began in the 1630s with the designation of the Congregational Church as the colony's "established church." The Puritan fathers enacted laws decreeing church attendance on Sundays and other appointed days, and requiring all residents to contribute to the financial maintenance of local Congregational ministers. Educational patterns, business practices, social conduct, and sexual activities were all comprehensively controlled in accordance with Puritan principles. "Blue Laws" provided penalties for offenses against God's word, such as profanation of the Sabbath and swearing, and capital punishment was mandated for adultery, sodomy, bestiality, lesbianism, harlotry, rape, and incest.

Connecticut authorities harassed and often persecuted such non-Congregationalists as Quakers, Baptists, and Anglicans. However, the church was weakened during the 18th century by increasing numbers of dissenters from the Congregational order. A coalition of dissenters disestablished the church by the Connecticut constitution of 1818. The final blow to Congregational domination came in the late 19th and early 20th centuries, with the arrival of many Roman Catholic immigrants.

Since World War I, Roman Catholics have been the most numerous religious group in the state. As of 2000, there were 1,372,562 Roman Catholics in 411 congregations. Mainline Protestants represent the 2nd-largest category of churches and include the United Church of Christ with 124,770 adherents in 253 congregations, the Episcopal Church with 73,550 adherents in 186 congregations, and the United Methodist Church with 51,183 adherents in 133 congregations. The estimated number of Jewish adherents was 108,280 and Muslims numbered about 29,647. About 42.1% of the population were not counted as members of any religious organization.