This article argues for critical reflection on diachronic and quantitative claims about increasing religious diversity. Influential scholars in several national academies have asserted there is “increasing religious diversity” in various locales. Many also make specific claims about religious diversity in the past to enable comparative claims about religious diversity in the present or even in the future. Yet, such claims are flawed methodologically (though less flawed alternatives are available). Moreover, neither the professional ethos of curatorial caution nor the pluralist politics commonly accompanying such scholarship are well served by this approach. Since the study of new or emergent religions also entails central historical conceits—for example, in debates over the proper meaning of “new” or over precise relations to “modernity”—this article offers a comparative perspective to conclude that scholars working in the fields of new religions studies and religious diversity studies should be more aware of the basic historical frameworks into which they place their multidisciplinary practices, practices that could be enriched and strengthened by further dialogue with historians.