This essay explores the transformation of Catholic thinking about the right to religious freedom. Throughout the first half of the twentieth century, Catholics vehemently opposed this right (Pope Gregory XVI called it "absurd and erroneous"). Church leaders claimed that in order to protect Catholic Church teachings on marriage, education, and worship, states had to enshrine them in law, even if that meant severely limiting the rights of Protestants, Jews, and others. The 1950s and 1960s, however, witnessed a sea change, as a growing chorus of thinkers and theologians called on Catholics to embrace a pluralist and tolerant attitude.