Abstract : Since the 1950s, the Cook Islands – a Polynesian State in association with New Zealand – have experienced a dramatic growth of internal migration – from the remote islands to the main island Rarotonga – and international migration: in 2006, 20,000 Cook Islanders lived in the Cook Islands and 58,000 in New Zealand. This article examines the impact of these migrations in terms of religious beliefs and practices, church organisation and cultural identities. In New Zealand and in the Cook Islands, migration takes place in a religious context characterised by a strong diversity, especially within Protestantism. This diversification of the religious offer raises the issue of “authenticity” of religious and cultural practices. It also implies new questions about the institutional management of religious pluralism.