Maryland was founded as a haven for Roman Catholics, who still make up the largest single religious group in the states although their political supremacy ended in 1692, when Anglicanism (now the Episcopal Church) became the established religion. Laws against "popery" were enacted by 1704 and Roman Catholic priests were harassed; the state constitution of 1776, however, placed all Christian faiths on an equal footing. The state's first Lutheran church was built in 1729, the first Baptist church in 1742, and the earliest Methodist church in 1760. Jews settled in Baltimore in the early 1800s, with a much larger wave of Jewish immigration in the late 19th century.

As of 2000, there were 952,389 Roman Catholics in Maryland. Adherents of the major Protestant denominations included United Methodists, 297,729; Southern Baptists, 142,401; Evangelical Lutherans,103,644; and Episcopalians, 81,061. In 2000, there were an estimated 216,000 Jews and about 52,867 Muslims. Though membership numbers were unavailable, reports indicate there were about 32 Buddhist congregations and 26 Hindu congregations in 2000. Over 3 million people (about 56.7% of the population) were not counted as members of any religious organization.