Archbishop Justin Welby’s statement to the Christian radio station Premier that he talks in tongues every morning is an appeal using one of the clearest markers to his evangelical tribe. Talking in tongues is a streaming babble of nonsense syllables delivered with a curious calm authority, as if what the speaker were saying made incontrovertible sense, even though it contains no recognisable words. It sound like what it probably is, the spontaneous operation of the speech-producing parts of the brain when they are completely decoupled from the bits that handle language and thought.

For believers, it is a priceless expression of the presence of the holy spirit. It is mentioned in the New Testament, and has been the defining feature of the charismatic movement, which was the major development of Christianity in the last century.

It is certainly what saved evangelical Christianity in Britain from the extinction that swallowed most traditional protestant churches in the past 50 years. When Christian doctrine makes no sense in the modern world, a retreat into nonsense may be the only way to save it.

Someone as driven as the daytime Welby might find babbling in the dark a deeply needed psychological relief

Speaking in tongues is not hard to do if you want to, and find yourself among people who do it themselves and expect others to. Perhaps the most intensive instruction is provided by the Alpha course, invented in Welby’s home church, Holy Trinity Brompton in London; there, on the sixth weekend, the participants are taken away to a residential retreat and most find themselves praying in tongues.

Someone as driven and disciplined as the daytime Welby might find in early morning babbling in the dark a deeply needed psychological relief. Permission from God to stand at ease must give a wonderful feeling of security. A friend of mine who was a missionary in her youth still remembers with joy the experience of God, as she felt it, praying through her as she sang in tongues at Taizé.

What is more weird is the notion Welby mentioned of “words of knowledge” – little nuggets of prophecy direct from God that work their way to the surface of the world like meteorites appearing from under the Antarctic ice fields. Prophecies have traditionally been one of the main forms of psychological manipulation within the movement. Charismatic leaders can use them to push their followers in desired directions; even more clearly than public, performative prayer, they can be used to get across messages that the messenger would not wish to stand behind.

It is hardly surprising that the first records of charismatic phenomena in the New Testament are contemporaneous with St Paul’s efforts in his letters to calm down all the excitement, and to insist that the people who had the visions were not also those who got to pronounce on their significance and meaning.

Again, the underlying phenomenon is remarkably common and the ability can be trained. Professor Tanya Luhrman’s wonderful book When God Talks Back, about her experience as an anthropologist in a charismatic church, describes how members there were encouraged to treat God as an invisible friend: pouring a second cup of coffee for him at a break, and so on. For people with a particular character trait, measured on the Tellegen absorption scale – which is, roughly speaking, the ability to climb aboard a train of thought and have it carry you far from the everyday world – this kind of practice can lead to a constant sense of supernaturalism in the world.

Welby rather wisely sidestepped the question of whether God talked to him as well as to some of his correspondents. His listeners on the evangelical radio will have taken it for granted, but no one in the outside world wants an archbishop who takes directions from God. That is altogether too much like religion and not what the Church of England stands for at all.

But the urge to divine the future goes back a very long way into prehistory. Oracle bones are among the earliest relics of Chinese civilisation. The cracks that appeared when they were burned in fires were held to form significant patterns. The Greeks and Romans read the future in the spilled entrails of sacrificed birds. Even today, the urge to find significance in the apparently random opinions of complete strangers is hardly confined to archbishops. It’s just that when journalists practise divination by taxi drivers, as foreign correspondents traditionally do, we call it a vox pop, not a vox dei.

• Andrew Brown writes on religion. His book Fishing in Utopia is a memoir about his life in Sweden