WRITING a biography is like doing a jigsaw. You assemble information from various quarters, and piece it together. But it is not a nice new shrink-wrapped factory-fresh puzzle. It is a second- or third-hand one and, as you proceed, you begin to suspect that there may be some bits missing.

In the case of my biography of Pope Francis, there was one piece missing at the end. I could tell from the parts around it what shape it must be, but I could only guess at the precise colour. In the 12,000-word interview the new pope gave last week to a consortium of Jesuit publications, however, he endorsed the deduction that I had made.

Everyone who knew him well in Argentina had told me the same story. He was a man who had changed - from a traditionalist authoritarian who had bitterly divided that country's Jesuits to the icon of radical humility in the Vatican today. That change, it was evident, had happened in only two years, when he reached the age of 50, after he had been stood down as leader of the Society of Jesus, but before he became an assistant bishop in Buenos Aires. But no one could tell me what prompted the transformation.

My surmise was that two things brought it about: deep prayer - for I had been told by those close to him that he made all his important decisions while at prayer - and immersion in Ignatian spiritual exercises during the two years when he was exiled to the second city of Argentina, Córdoba. He had been sent there after he had been unable to resist meddling in his successors' affairs in the province, colleges, and Jesuit houses in Buenos Aires, when his 15 years as novice master, provincial, and rector were over.

So it was with satisfaction, and a certain relief, that the conclusion I had drawn from the circumstantial evidence was confirmed by the man himself. "My style of government as a Jesuit at the beginning had many faults," the Pope told his interviewer, Antonio Spadaro SJ. "I found myself Provincial when I was still very young. I was only 36 years old. That was crazy. I had to deal with difficult situations, and I made my decisions abruptly and by myself. . . It was my authoritarian way of making decisions [that] led me to have serious problems and to be accused of being ultra-conservative. I lived a time of great interior crisis when I was in Córdoba."

Discernment, he revealed, was the key to resolving that crisis, and to getting to know God and follow him more closely. The vision of Ignatius - which Jesuits are brought to confront in the 30-day silent exercises that are repeated throughout their formation - was, he said, vital to his pondering on the issue of different roles in the government of the Church and the challenge of being placed in authority over others.

"Over time, I learned many things. The Lord has allowed this growth in knowledge of government through my faults and my sins." The great authoritarian as Provincial became the greater consulter as bishop, then archbishop, in Buenos Aires. Consultation and participation enabled him "to make the best decisions", he concluded.

His interviewer did not probe further, which is a shame - for this, it seems to me, is the key to understanding Francis and the qualities that have the potential to mark him out as a great pope. His humility and authenticity grow out of the pain of having made grave errors and having learned from them. "I am a sinner," he told his interviewer. "It is not a figure of speech, a literary genre. I am a sinner." So are we all, and so, perhaps, great things can be expected of all of us, too.

Paul Vallely's book, Pope Francis - Untying the knots, is published by Bloomsbury.