Few outside Argentina had heard of Archbishop Jorge Bergoglio when he was elected Pope last year and even fewer had thought of him as papabile. Eighteen months later, while the world is in love with this humble, intelligent charismatic Jesuit priest and Pope, conservative Catholics are muttering words such as "schism".

The extraordinary synod of bishops in Rome last month is regarded by many as an outstanding success, the start of a truly exciting and transformative process not just for the Church but for Christianity worldwide.

Yet it is seen in some traditionalist circles as a disaster, in particular for Pope Francis himself. Some are left unhappy and even bewildered by the scale and pace of change.

Far from heading for schism, it could be that Pope Francis is bringing in a revolutionary new pastoral theology that is likely to see the Church slough of its anti-sex, anti-gay rhetoric for a more creative, understanding approach, a change already being implemented without needing a word of doctrine to be amended. He is also leading the Church towards greater unity with the wider Christian world, in particular the charismatic evangelical movement.

An in-depth picture of Jorge Bergoglio is drawn in a new book to be published later this month, already being spoken of as the definitive biography of this Pope. The Great Reformer is by Austen Ivereigh, the British journalist uniquely fitted for the task by an Oxford University doctorate that was an investigation into the Church and politics in Argentina between 1930 and 1960.

A fluent Spanish speaker, this expertise gained him unrivalled access to the Jesuits in Argentina during his research and enabled him to read everything Pope Francis has ever written. When he went on to attend and report on last month's synod, he had possibly the greatest understanding of any lay person there of what was actually going on, why, and what it would mean.

"What we have is a Pope who is, as somebody said to me in Argentina, a combination of a brilliant manager and a desert saint," he said. "He's machiavellian plus St Francis. He really understands government, he understands management. But above all he understands spirituality. And this is what's just so unusual about him."

Speaking exclusively to Christian Today, he said that if there is schism, it is more between how the world views the Pope, and how he is seen in Rome.

"It is always striking to me when I go to Rome to find out that the enthusiasm for Pope Francis which is phenomenal, remains phenomenal in the wider world, isn't particularly shared in Rome," said Ivereigh. "In fact you talk to Vatican officials and they look very worried, they say, 'We don't understand what's going on, we don't know what he's doing, we can't make him out.' I know from my biography of Francis, from having spoken to many many people who knew him and worked with him in Argentina, that he is a disconcerting leader and he doesn't operate through institutional channels."

Pope Francis prefers to work at a personal rather than institutional level. "He's still in a period of wanting to shake things up. He is shaking things up in Rome. So actually most of the department heads don't know whether they are still going to be there or not in a few months' time. In fact most of them I think are fairly confident that they won't be. So that produces a kind of uncertainty and insecurity. But he's not afraid of that. Here's the thing about Francis: He's not afraid of living in the tension, and he believes that in the tension is actually where the Holy Spirit is allowed to operate."

Ivereigh also revealed that one of the most extraordinary things that is happening under Pope Francis is the beginning of an historic breakthrough between the Catholic Church and evangelicals worldwide.

This came about because of the Pope's friendship with the late Bishop Tony Palmer, a charismatic bishop of Communion of Evangelical Episcopal Churches. Jorge Bergoglio had become close to him many years ago in Argentina. Ivereigh interviewed Palmer for his book before he was killed tragically in a traffic accident earlier this year.

The Pope had already sent a video message via Palmer's mobile to the Charismatic Evangelical Leadership Conference hosted by Kenneth Copeland that went viral. At a subsequent meeting in June, evangelical leaders representing millions of Christians worldwide discussed a draft of a declaration to be made public in 2017, which will affirm that Catholics and Protestants are no longer rivals but are committed to working together in mission.

Palmer's widow Emiliana and Archbishop Robert Wise of the Communion of Evangelical Episcopal Churches are committed to taking this forward. "I am sure that one of the legacies of this pontificate will be this historic declaration," said Ivereigh. "Francis obviously wants to see unity between Christians, however it happens. But he knows that institutional unity isn't going to happen. There are too many obstacles. With evangelicals, with charismatic Catholics, with people who haven't got that institutional baggage, they are capable of witnessing together, praying together, working together, in a way that perhaps isn't possible with some of the older churches. So I think we are going to see a major breakthrough there."

The declaration will include an agreement to pray and witness together. "It is liberating Christians to [do] mission together. It is important in a cultural context where there's greater hostility to faith than ever before that Christians can unite in way that wasn't conceivable even 100 years ago," said Ivereigh.

Likewise, while institutional breakthrough is, frankly, unlikely between the Church of England and the Catholic Church, there is a great meeting of minds between Archbishop of Canterbury Justin Welby and Francis. "These are people who start with the Holy Spirit. To understand Francis, he's a Jesuit so he comes with that spirituality. He's a Jesuit who discovered late in his life charismatic spirituality. And that's in many ways the key to understanding him."

The Pope has in less than two years changed the narrative about Christianity in Western culture, and it is no longer assumed that this is an unchanging institution locked into the past and incapable of reforming itself. "People are seeing the Church in a different light than they did a couple of years ago," said Ivereigh. "Simply by being he has made real and made evident the Church's compassion, the Church's mercy, the Church's love. People just see in him a deeply loving, humble human being."

Ivereigh believes the changes already wrought by Francis are irreversible. "Even though Francis may not last long, he himself has said he's probably only got a couple of years, I think the next few papacies are going to be shaped by this one. That's why I haven't hesitated to call my book The Great Reformer. I think he will be known in history as one of the great reforming Popes in the Catholic Church."

In spite of the body of bishops deeply unhappy with the direction of the Papacy and the Church under Francis, Ivereigh believes that 80 or 90 per cent of the residential bishops from dioceses around the world back the Pope.

"They want to explore how to make the Church a more pastoral place, how to make Church teaching fuller so it includes more of the merciful love of God without compromising essential Church teachings."

Ivereigh helped create Catholic Voices in 2010 to put the Church's case on television and radio. It consists of teams of people, ordinary lay Catholics, not theologians, so they are a good guide to what people in the pews are thinking. "I think they want to see a Church which is more missionary, more effective at getting its message across. They want to see a Church also which is attractive. And they also want at a very basic level to try and get rid of or move on from those things which have kept the Church back, namely certain scandals and dysfunctions, whether it is in the Vatican or the clerical sex abuse crisis."

So can he be stopped? Will he be another Barack Obama, as one conservative commentator suggested this week? Ivereigh thinks not. "He actually understands power very well and he believes in it. And he said to the bishops at the end, 'What are you worried about, you are with the Pope, you are cum Petro et sub Petro, with and under Peter. He was kind of challenging them, because when the bishops are gathered with and under Peter, the Church has the assurance of the guidance of the Holy Spirit. So he's saying, what are you worried about, the Pope is here. And he also said, and I know he would have had a big smile on his face when he said this, the Pope has supreme universal power in the Church. He was using a canonical formula which says, 'The buck stops with me. I have the power.' So yes he understands power, yes he has the power."

Change has already happened on the three hot-button issues - a path back to the sacraments for the divorced and remarried, homosexuality and contraception. "What happened during the synod is that a new language was used about all three, but particularly the divorced and remarried and homosexuality. That new language is here to stay. So there has been a shift already. It is never going to be enough in the future for the Church to say about homosexuality, it is intrinsically disordered. The Church has to start with Francis' famous question on the plane coming back from Rio de Janeiro, 'Who am I to judge?' These are God's children. We start with that."

What Ivereigh believes could happen on sexuality is an agreement that the different local churches not only have their own language but their own pastoral strategy.

He also thought it likely that bishops will be given the power to decide locally, on a case-by-case basis, whether people should be re-admitted to the sacrament and that the annulment system will be made more accessible and efficient.

Also at the synod was Christopher Lamb, news editor of The Tablet, the Catholic newspaper. It was Lamb's second synod. He attended the 2012 synod on evangelisation where, he says, it was difficult to get interviews and there was "a bit of a sense of crisis in the Church" around issues such as Vatican leaks. This time the experience was completely different. "This gathering was an amazing experience to cover. There was real debate, a sense that the bishops and cardinals could speak freely. That's what the Pope asked them to do and they did it. It was quite exciting to see this in action."

He sees the main issue in the pews as being how to support marriage and family life, which is in a lot of trouble in many parts of the world. In Africa, people might think everyone's following Catholic church teaching, but they have their own problems with issues such as polygamy and child brides, while in Western Europe there are difficulties with the financial pressures on married couples and families.

"The synod fathers wanted to find new way of making the Church's teaching more attractive. A certain language has been used over the years, especially at the level of the Vatican, which has been a little bit harsh. That came through very very strongly from the gathering. There is a sense that there is this disconnect between what the Church teaches and what is lived. That was a big concern."

The process will move forward at the next synod in October 2015. The Church needs at that point to decide what is going to happen in terms of this new language and how the Church speaks to Catholics living not in conformity with Church teaching. "I think we will see a new language, a new approach, I think we will also see more and more debate, I think we'll see more and more disagreement, because the Pope has opened up this discussion. It should be pointed out though that he does still hold the cards. The synod is only a discussion body. It is not a decision-making body, it's not like the General Synod of the Church of England. So the Pope will decide what eventually is produced from the synod."

Lamb spoke to some of the cardinals who weren't happy with the way the discussions went at the synod. Australia's Cardinal George Pell, current Prefect of the Secretariat for the Economy in Rome, was among them. "I interviewed him a couple of days after the mid-term synod report which was very welcoming to gay couples. He was not happy and he gave to me a piece of paper with his handwritten thoughts about where the synod document was going wrong. He words were that it was 'tendentiously incomplete'." But Pell's position is nuanced. "His concern is that if the Church kind of gives in to secular values of the world, it's not doing its job, that there's a prophetic role for the Church. The Church should shape culture rather than be shaped by the culture."

At the root of the changes being attempted by the Pope is the need to make the Church teaching better understood and expressed. "There is a very real problem in that you have Church teaching that's not lived and that the vast majority are struggling to follow. Let's take the whole question of the divorced and remarried. Many divorced and remarried Catholics are receiving communion. They feel they should be able to receive. That's a problem for the teaching if it doesn't have the credibility. The Church has to respond to that. It can't just give in. It can't just change it according to the whims of the world, but it has to respond to the pastoral realities."

Lamb believes the Pope is taking a risk. "He is saying to the Church, the bishops, we are going to have a discussion about this. He is staking some of his credibility on this process working, and if it doesn't work, there could be very serious consequences. Having said that, I think it will work because it's already working. We've already got this change in language, we've already got this debate."

He did not anticipate any attempt to oust Francis.

"There are obviously people who are not happy. I notice on some conservative blogs, occasionally they talk of antipopes and you see pictures of Benedict XVI. But I don't think there is any suggestion that Francis will be got rid of. I think he is a very very canny operator. You can't be Archbishop of Buenos Aires for many many years without knowing how things work politically. I actually think this synod has almost given him more respect."

The next year, leading up to the last synod in this process of reform, will be key. "You will see lots of books, interviews, pamphlets, putting forward different views. It will be a very exciting time." And that, surely, will be a good thing for both Church and world.