The largest religious fact about Britain is not a domestic one. From the eighteenth century the island's churches built an apparatus for sending people abroad: the Society for the Propagation of the Gospel, the Baptist Missionary Society, the London Missionary Society and the Church Missionary Society among many others, funded by subscription and staffed increasingly by tradesmen and their wives rather than by clergy alone.
The relation between that effort and the empire was never simple, and Andrew Porter's study of it argues against reading missionaries straightforwardly as agents of expansion. Missionary societies were often at odds with colonial administrations and with settler interests, campaigned against slavery and sometimes against the taking of land, and worked in places the empire did not govern. They also travelled on the empire's ships, relied on its protection, and carried assumptions about civilisation that were not separable from it.
What is not in doubt is the result. The largest Anglican provinces are now in Africa, the churches planted by British societies in India, the Pacific and the Caribbean passed into local hands long ago, and the theological arguments that most divide the Anglican Communion today run between those provinces and the church that sent the missions. Attendance in Britain has fallen while the communion it seeded has grown, which reverses the direction of authority and is one of the more consequential religious shifts of the last century.