The bewildering influx of immigrants into London has had one effect that no one could have predicted 20 years ago: it's making our capital city religious again.

We've noticed – but only up to a point. Islam is visible: the women in niqabs, the new mosques, the Halal butchers. But the transformation of Christianity in London is harder to spot. If you asked the average Londoner how many Sunday churchgoers in the city were black, I suspect he or she would be startled by the answer: about half of them.

My guest on this week's Holy Smoke podcast is Ben Judah, whose knowledge of the demography of London was picked up by living in and among immigrant communities while researching his eye-opening book This is London.

Pentecostalism – especially of the West African variety – is by a wide margin the most dynamic Christian presence anywhere in Britain. Congregations spring up all the time – sometimes in buildings that are recognisably churches, but also in secular buildings ranging from office basements to converted warehouses.

Judah's insights into this community are fascinating. Disturbing, too, if you prefer your Christianity to be polite and self-effacing. Hundreds of Pentecostal preachers flavour their evangelism with promises of miracle cures and sudden financial prosperity. But do their noisy followers actually believe these promises?

Also, how many of London's Muslims really subscribe to the jihadist messages with which they are bombarded by fanatical Islamic 'religious entrepreneurs'?

You'll have to listen to the podcast to hear Judah's thoughts. But here's one fact that surprised me: London, headquarters of Britain's easy-going metropolitan elite, now has fewer births out of wedlock than the rest of the country. Why do you think that is?

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