Justin Welby is working in Thomas Cranmer’s old study in Lambeth Palace, a room that looks as if it hasn’t changed at all since the Book of Common Prayer was written here almost six centuries ago. It feels like a mini-monastic retreat: there is a desk, a crucifix, several Bibles and not much else. The 105th Archbishop of Canterbury studies and prays here, deciding how best to lead a national church whose Sunday services are now attended (according to its own figures) by barely 1 per cent of England’s population. These are new times — and require new tactics.

When he was enthroned six years ago, he was seen as just the man to provide the new tactics. He’s a convert, who was ordained in his thirties after a career as an oil company executive. He has literally risked his life for his church when working as a missionary in Niger Delta: a militia leader ordered him to be taken outside and shot, but he was saved by the intervention of a local elder. His current job is less dangerous but by no means easier. It requires huge reserves of strength, imagination — and optimism.

‘The decline is flattening,’ he tells me. But to understand the modern Church of England, he says, you need to look at the far-larger ‘worshipping community’. ‘Churches across England are now involved in more than 33,000 social projects. Food banks, night shelters, debt counselling, family ministry — all kinds of other things.’ Since the crash, he says, the Church of England has launched into all kinds of social action helping those affected – food banks especially. He seems almost offended when I ask if it really counts as religious activity. ‘Feeding the poor? I think Jesus would have thought of it as a form of religious activity.’

The bright spot, for him, is vocations — which he says will soon be at a 40-year high. That is striking, given that church numbers are at an all-time low. I ask why this might be. ‘You will think me very naive and sort of naff about this, but I think it’s probably got something to do with God. We’ve made a real effort to pray for and encourage vocations and we’ve seen a very significant rise, getting on for well over 20 per cent over the past three years.’

He accepts that, overall, the numbers are challenging: vocations are rising but weekday and Sunday services decreasing and the number of marriages and baptisms declining sharply. Perhaps the most startling statistic is that just 2 per cent of under-25s regard themselves as Anglican. ‘If you’re over 70, you’re eight times more likely to go to church than if you’re under 30,’ he says. ‘And I think that’s a huge challenge.’ I ask if he thinks rising secularism is also a challenge: that young people who go to church are seen not just as weird but as potential bigots and homophobes. It’s not a story he recognises.

‘Let’s get back to the 1980s, when I was in the oil industry,’ he says. ‘Christians I worked with would have experienced something very similar to that and we kept our heads well down. We wouldn’t have been accused of being homophobic, but of all kinds of other things. A crazy attitude to sex, for example, if you didn’t believe in sex outside of marriage.’ Headwinds, he says, may have strengthened a little. ‘But I think there’s a danger of Christians being overly fearful and too defensive.’

When it comes to politics, the Archbishop is neither. He has been quite busy recently, and was given a standing ovation at the Trades Union Congress after laying into Amazon (for having ‘leeched off the taxpayer’); denouncing zero-hours contracts (‘evil’) and the rich (he quoted the Song of Mary to the effect that they should be ‘sent away empty’). When he also joined an advisory panel put together by the IPPR, John McDonnell’s favourite think tank, it seemed to some Tories that the Archbishop was making a home for himself on the political left.

Not at all, he says. 'I would be worried if I thought my analysis was based on a secular economic model or a Marxist model. Or, for that matter, a Keynesian model, a Friedmanite model or a Chicago Boys kind of thing. I have to keep going back to the Bible: to the prophets and the Gospels.’

But he finds no shortage of radicalism in the Gospels. ‘One of those wonderful archbishops in Latin America, who I wouldn’t presume to emulate, once said: “When I feed the poor, they call me a saint. When I ask why they are poor they call me a communist.” That seems to me to set up the dilemma rather well.’

The dilemma doesn’t stop there. Take Amazon: if it’s so immoral, why does it rank in the Church of England’s top share holdings? ‘Well the church’s approach to investment has been, on the whole, to hold shares so that you have a voice and can engage.’ So is this the rationale: to invest in companies to leverage influence though sheer size of shareholding?

‘No it’s not. I mean obviously if we had sort of 20 per cent — chance would be a fine thing! But there is a willingness in these companies to listen to the church.’ A few weeks after his speech, he said, Amazon ‘put up their wages by about 30 per cent to what we call the living wage’. He put this down to investor pressure, not the power of the Welby sermon, but it shows how companies respond to such pressure. ‘It’s very easy to stand on the sidelines and condemn the players on the pitch. But that doesn’t seem to me to be the right way to behave. It’s much better to be involved. And to be challenging from within it, from within the game and pushing from within.’

His dissertation at theological college was entitled ‘Can companies sin?’ His conclusion: very much so. In his new job, he is elaborating on this point. His quote about sending the rich away empty, he says, is part of the Song of Mary and, as such, an essential part of the Christian message. ‘The Magnificat is not just a background part of the Bible. It’s not just a minor bit. It is one of the most fundamental statements. Look at the Nazareth Manifesto where Jesus himself quotes from Isaiah about his social engagement. That, again, is radical theology. The East India Company banned the saying of the Magnificat in evensong because they knew how radical it was.’

To speak it in church is one thing, I say, but to quote such passages at the TUC conference — isn’t that getting rather political? ‘So the Bible’s only safe within church? That’s a very striking idea. That’s quite radical, I’d have said. Shall we just lock Christianity away?’

The bishop he was referring to earlier was certainly a radical: Hélder Câmara, an early advocate of liberation theology. He was also a Roman Catholic. Not so long ago, it would be unthinkable for the Archbishop of Canterbury to be quoting agents of Rome, but times change. The two churches have been holding talks about possible reunion since 1970, but since the C of E admitted female vicars their paths have tended to diverge. Ten years ago, the Vatican made it easier for vicars to defect to Rome. Hundreds did so and now, by some estimates, one in ten Catholic priests is a former Anglican vicar.

I ask what he thinks about all this. ‘Who cares?’ he says. ‘I don’t mind about all that. Particularly if people go to Rome, which is such a source of inspiration. I had an email from a very old friend, an Anglican priest who has decided to go to Rome. I wrote back saying: how wonderful! As long as you are following your vocation, you are following Christ. It’s just wonderful. What we need is for people to be disciples of Jesus Christ. I don’t really care whether it’s the Church of England or Rome or the Orthodox or Pentecostals or the Lutherans or Baptists. They are faithful disciples of Christ.’

If you think this is an unusual thing for the Archbishop of Canterbury to say, then you don’t know Justin Welby. He is a bridge-builder, so keen on fostering greater unity amongst Christians that he has assembled in Lambeth Palace a group of young Christians of various denominations called the Community of St Anselm. ‘One of the prayers we say every morning is for the unity of the church. That seems to me to be much more important. God called the church into being. We, as human beings, have managed to mess that up and split it up.’

I grew up Catholic in a part of Scotland where such divisions mattered more than they ought to, at a time when it was unthinkable for an Archbishop of Canterbury to describe defections to Rome as ‘wonderful’. But it’s hard to imagine a less sectarian cleric than Justin Welby. He has a Catholic priest, Fr Nicolas Buttet, as his spiritual adviser. One of his closest friends, he says, is Vincent Nichols, the Catholic Archbishop of Westminster.

‘Cardinal Nichols and I would describe each other as very close friends. We see each other regularly, we pray together, we talk together,’ he says. ‘You know, 50 years ago, that would have been news.’ And he has Catholic friends in even higher places. ‘I go and see the Pope quite regularly. We talk about personal things,’ he adds, ‘about what it is to be a follower of Jesus Christ in today’s world. I ask him questions, and he is very helpful.’ He sounds so enthusiastic that I ask if he has ever been tempted to make the jump. ‘I think that might cause a little bit of upset,’ he laughs. ‘Even nowadays.’

Perhaps one of the most famous Anglican bridge-builders was George Bell, who as Bishop of Chichester sought to unite churches against the Nazis. Six years ago, a woman said she was molested by Bishop Bell as a child. Given that he died in 1958, investigation was difficult but the church still issued an apology and compensation. This soon led to an outcry and complaints that Bishop Bell’s name had been besmirched without any proof. Many Anglicans blame the Archbishop himself for mishandling it all.

‘It has been a very, very painful process,’ he says. ‘Not least because Bishop Bell was — is — one of my great heroes.’ He sees the fiasco over the investigation as part of a far wider problem. ‘Probably the greatest failure of the C of E since the second world war has been our failure to deal adequately with disclosures of abuse. When I came into this role, I didn’t have any idea how bad it was.’

At the time, he says, he had just one person working half-time for the Church of England, nationally. ‘Our budget nationally was £50,000 a year, £100,000 a year, or something like that. It’s now about £7 million. I am still very uncomfortable with the system. I think it is slow. We have not yet found the proper way of dealing properly with complainants and taking them seriously, listening to them, not telling them to shut up and go away, which is what we did for decades. Which was evil. It’s more than just a wrong thing: it’s a deeply evil act. At the same time we have not found a way of caring for those who have been accused or complained against — or their families.’ Bishop Bell, he says, is the ‘pre-eminent case’ that shows how the church didn’t ‘do it right’.

Sussex police took a few weeks to investigate and dismiss the case against Bishop Bell. The C of E’s investigation has taken two years so far. Can’t he speed things up? ‘I can’t, because it’s independent. That would be interfering.’ And the church buildings where Bishop Bell’s name has been removed? ‘I understand why people did that, in the present atmosphere. We’ve got to find a proper way of acting promptly and doing justice and we haven’t found that yet.’

The man I meet in private is rather different to the Archbishop Welby of recent headlines. In his political interventions he has sounded combative, even bombastic. In conversation, he’s quick to admit that such issues are seldom clear-cut. Most zero-hours contracts, he tells me, are fine. You can ‘argue both ways’ about turning back refugee boats if it leads to fewer deaths. His criticism of welfare reform doesn’t undermine his sympathy with the overall agenda. And while he is against Brexit, he would never claim God is on one side or the God is on one side or the other.

‘These are not issues that define your Christianity,’ he says. ‘Christianity is just defined by faith in Jesus Christ.’

Now listen to Peter Hitchens go head to head with Damian Thompson on the modern Anglo-Catholic split, in the latest Spectator Podcast.