Abstract The Dutch neo-Calvinist mission (Zending) on Sumba in Indonesia, an area known for its Marapu ancestor religion, started in 1902. The independent Sumbanese church (Gereja Kristen Sumba, GKS) was established at the 1947 Synod. One century later, by 2002, two out of three inhabitants were Christian. The research question is whether the rise of a vital GKS was facilitated by education and ‘antithetical’ notions of freedom offered by neo-Calvinist Zending. The answer is that the Zending empowered Sumbanese Christians to decide for themselves whether to preserve traditional customs (adat), so they could build the GKS from the bottom up. This answer is based on archival material – including the unexplored archive of Rev. P.J. Lambooij – and on a dozen semi-structured interviews with Sumbanese spokespersons in 2006, 2016 and 2019. Macro-level explanations – capitalism, modernity, or colonialism – hardly appear to account for the transformation to Sumbanese Christianity.