This study documents the emergence of pluralism as a constituent dimension of the culture of American Catholicism, and its implications for religion in public life. When such "internally grounded pluralism" is a factor in the relations between religious groups in a society, it supports a larger civic culture in which the substantive content of particular religious traditions may be brought into shared, public arenas. Such a proposition runs counter to the tenets of the classical sociology of religion, especially those associated with secularization theory. According to long-running assumptions in the field, religious diversity in modern societies leads to the public delegitimation of all religions, and hence to their privatization. This proposition also runs counter to the common sense of Western modernity, broadly speaking. To the extent that modern societies accord equal place to a range of faiths, none have a particular claim on public life. Therefore, the normative expectation is that public life will be denuded of religious reference. However, many contemporary American religious groups have in fact begun to operate under a more open religious paradigm, one unanticipated by classical notions and secularization theory. And this cultural innovation has also had unanticipated implications for the place of religion in public life, calling the older model and its corresponding assumptions into question. This analysis shifts the focus from the macro-level fact of religious diversity within a society to the intermediary level--the level of religious groups, their culture, and their mutual relations. Its claim is a particular instance of a more general proposition: That the larger regime governing religion in public life is always a function of the relations between religious groups. Since ostensibly, the norms governing American public culture are a function of purely neutral, secular considerations, this proposition often goes unacknowledged. This analysis proceeds by developing the notion of internally grounded pluralism; sketching the history of interfaith relations in America; and then showing how religion in public life reflects these relations. In contemporary religious environmentalism, one locus where internally grounded pluralism has emerged in Catholic culture, we witness religious initiatives with considerable public reach despite their religious content