This article discusses the intertwined histories of Christian ecumenism and ethnic nationalism in Swaziland, shedding new light on the importance of Protestant evangelical thought in the formation of nationalist ideology in 20th-century southern Africa. From the 1920s to the 1970s, Swaziland’s Zionist Christians sought to amalgamate Swaziland’s churches under the auspices of a ‘biblical Christianity’ that harked back to the Apostolic era. Their ecumenical project was strategically harnessed by the Swazi Paramount Chief, Sobhuza II, who discerned the utility of propping up embryonic Swazi nationalism by the formation of a single national church. While Zionist ecumenists and Swazi nationalists sought to create a unified church for a unified nation, the success of their collaborative project was always limited. As well as their desire to unify across denominational borders, Zionist Christians were also characterised by a quintessentially Protestant urge to follow the prompting of the Holy Spirit in breaking away from existing churches and forming disparate new organisations.