Inculturation is the process by which a religion, in Christian usage, the gospel, takes root in a particular culture and comes to express itself in that culture's forms. The term is Catholic and twentieth-century, becoming standard after the Second Vatican Council, and it is distinguished from the older "adaptation" or "accommodation" by claiming a deeper and mutual process: the culture shaping the expression of the faith, and the faith transforming the culture.
The underlying question is as old as the religion. Acts 15 records the first and decisive instance, whether gentile converts had to be circumcised and keep the Jewish law, and the decision in the negative is what made Christianity portable in a way it would otherwise not have been. Every subsequent expansion raised the question again.
Its most instructive episode is the Chinese Rites controversy. Jesuit missionaries in China from the late sixteenth century, Matteo Ricci foremost among them, adopted Chinese dress and learning, used existing Chinese terms for God, and judged the ancestral and Confucian rites to be civil honours compatible with Christian faith. Dominicans and Franciscans disagreed, holding them idolatrous. Rome eventually ruled against the Jesuit position, decisively in 1704 and 1715 and again in 1742. The Kangxi emperor responded by restricting Christian missions, and the church in China was set back for two centuries. The rulings were substantially reversed in 1939, and the episode is now generally treated within the Catholic Church as an error and serves as the standing cautionary example in discussions of inculturation. The parallel controversy over the Malabar rites in India followed the same course.
Modern practice has gone considerably further. Vernacular liturgy after Vatican II; African and Indian liturgical adaptations; the use of local music, art, architecture and gesture; and the Zaire Use of the Roman rite, approved in 1988, are the visible results. Theological inculturation, using local philosophical vocabularies to express Christian doctrine, as African, Indian and Latin American theologians have done, goes further still, and African episcopal conferences have made it a sustained priority.
The limits are where the argument sits. Critics warn of syncretism, of adopting practices carrying meanings incompatible with the faith, and of a relativism in which the gospel becomes whatever a culture already believed. Advocates reply that the alternative is not a culture-free Christianity but an unacknowledged European one, and that Greek philosophical vocabulary and Roman legal categories were themselves inculturation, since naturalised by long use and no longer recognised as local. Both observations are correct, which is why the question of where the line falls has never been settled in principle and is argued case by case.