Calvin’s ecumenical passion has often been documented by scholars (section 1). This article argues that these practical en- deavours were based on Calvin’s ecclesiological conviction that there is only one church (section 2), based on the communio of believers with Christ and with one another (section 3), in which the sacraments, particularly the Lord’s Supper, play a crucial role as the bonds of communio (section 4). For Calvin, this concretely implied that believers who participate in the cele- bration of the Supper should also love one another, as quota- tions from the 1559 “Institutes” powerfully demonstrate (section 5). Given the ambiguous legacy of Calvin in South Africa, these views of Calvin present a continuous challenge to South African churches in his tradition (section 6).