This article presents ethnographic material and anthropological insights on the development of Islam in Vanuatu, where it has been studied even less than elsewhere in Melanesia. In Papua New Guinea, the Solomon Islands and Vanuatu, where dominant collective identities are firmly rooted in Christianity, converts to Islam remain few in number. In Vanuatu the Church authorities have nonetheless felt the need to reassert Christianity's hegemony nationally as a counter to the supposed advance of Islam. Also kastom, a widespread term for a specific local traditional heritage and an original way of living, has once again been invoked in recent debates on the politics of religious identity in Vanuatu. The aim of this article is to describe the conflicting arguments that people use in conceptualizing the moral or political aspects that show Islam to be culturally compatible or otherwise with Christianity and kastom on the island. I will examine how categories of continuity and change come to reinforce ideological discourses on the ability of Islam to conform to past Melanesian experiments in indigenizing foreign cultural influences. The case of Tanna in the south of Vanuatu will receive special attention. On this island, millenarian visions of a New World Order compete with local Muslims' interpretations of Islam. Older kastom social movements, having already experimented with ruptures from Christianity, like the famous John Frum movement, also participate in the construction of discourses on the continuity between kastom and Islam.