Otto Semmelroth played a major role in advancing the notion of the church as sacrament at Vatican II. His preconciliar works as well as his participation in working groups and committees were instrumental in introducing this systematic concept into the 1963 draft of Lumen gentium. His commentaries on the document disclose how his own understanding of the historical and eschatological dimensions of the church as sacrament was enriched through the process of developing the final 1964 text. Semmelroth’s nuanced treatment of this progressive theme enables him to serve as a mediating figure in the continuing ecclesiological controversies of today.