A marked feature of the contemporary U.S. constitutional landscape is the campaign by an Evangelical-Catholic coalition against the idea of secularism, understood by this alliance to mean the exclusion of religion from the state and its progressive marginalization from social life. Departing from the tendency to treat this project as a national phenomenon, this article places it within a longer global genealogy of an earlier international Christian ecumenical effort to combat secularism. The triumph of that campaign culminated in the making of Article 18 of the Universal Declaration of Human Rights, now considered the paradigmatic international legal provision on religious liberty. Article 18’s protection of the rights to proselytize and convert, I argue, was a product of an impassioned contestation between an ecumenical movement keen on securing the prerogative to spread the gospel to the non-Christian world and a secularism in a strange alliance with Islam in the region that held the greatest promise for the evangelical enterprise—Muslim Africa. In excavating the genealogy of ecumenical thought as it developed a critique of the so-called secularist threat, I recover the delicate links between the contemporary U.S. anti-secular campaign and the earlier ecumenical efforts.