This essay explores four main points in outlining the changing relation between ecumenism and interfaith engagement. First, it describes an ironic shift: Where once world mission was the common motive for Christian ecumenical engagement, now differences among Christians over response to religious diversity are themselves of church-dividing status. Second, it argues there is a new urgency for ecumenism as the necessary resource for adequate engagement with the religions—a new way for religious diversity to motivate ecumenism. Third, the essay traces the development of comparative theology as the appropriate theological resource for this new engagement. Fourth, it describes the convergence toward a trinitarian theology as a common element that marks both the recent ecumenical movement and the newer response to religious diversity.