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Statistics never tell the whole story when it comes to matters of faith, but the figures dealt with by Roy Jenkins' guest on All Things Considered this week (Sunday 29 April 8.31am, repeated Wednesday 2 May, 6.30pm) are impressive by any standards.

The body he runs brings together more than 340 churches, denominations and fellowships in 110 countries, with a total membership of more than 550 million.

Dr Samuel Kobia, a Methodist minister from Kenya, is General Secretary of the World Council of Churches, established to promote visible Christian unity, mission and service, but occasionally attracting political controversy as well.

He was one of the leaders of the Programme to Combat Racism, which aroused wrath in apartheid South Africa and in parts of Europe. He chaired peace talks in Sudan, helped reorganise the Zimbabwe Christian Council after independence, and has had strong words for governments on everything from the war in Iraq to nuclear proliferation and global migration.

During his visit to Britain this week he talks to Roy about the problems surrounding global migration, the changing face of Christianity in the west, and the challenges facing the ecumenical movement.