At a pavement cafe on the ceremonial street that sweeps from the Tiber to St Peter’s Square, two young German-speaking priests sized each other up one morning in the autumn of 1962.

Professor Hans Küng, 32 and Swiss, was destined to become the most famous dissident theologian in the world. Professor Joseph Ratzinger, a 35-year-old Bavarian, was to become the Vatican’s doctrinal inquisitor and, in time, supreme pontiff of the Catholic church.

Küng was informal, forceful. Ratzinger had a shy smile and what one acquaintance described as “a cruel mouth and come-to-bed eyes”.

“For me he was more timid with an invisible spiritual anointing, whereas to him I perhaps seemed audacious with more worldly charms,” Küng recollects today. We do not know what Ratzinger thought of Küng; although decades later his judgment would become clear: Küng should not be allowed to teach Catholic doctrine.

They were in Rome to advise the German-speaking bishops at the reforming Second Vatican Council. Evenly matched in scholarship and intellect, they were at the time both progressives, determined to curb bullying dogmatism in the church.

Ratzinger caused one of the biggest upsets of Vatican II with a bold speech challenging the power and secrecy of the Holy Office (responsible for Catholic doctrinal orthodoxy). He said: “In many respects the mode and procedure of the Holy Office . . . does damage to the church and for many is a scandal.” The old guard was outraged; progressives applauded.

More than 40 years on, Küng says: “This was the same Ratzinger who one day would become head of the inquisition authorities and who then, according to his own testimony, every day would receive top secret information from every continent, and of course send back top secret instructions to every continent . . . who himself time and again would give rise to the same culpable scandal.”

While researching a book in the Vatican not so long ago, I used to pass Ratzinger most mornings as he made his way across St Peter’s Square.

He lived until last week in a modest block of flats and walked alone to his office, indistinguishable from other Vatican clerics — bagarozzi, black beetles, the Roman children call them — in a black raincoat and a worker’s beret.

It was hard to believe this mild-looking prelate was the most powerful cardinal in the Vatican, controlling the doctrinal integrity and discipline of the Catholic church.

Last Tuesday he stood on the balcony of St Peter’s greeting the world as Pope Benedict XVI with a hands-together sporting gesture — I’ve won! I’ve won! Vatican City is rife with rumours that the liberals could not agree on a single candidate while the traditionalists unswervingly supported Ratzinger from the outset, giving him 100 votes by the third ballot, well above the two-thirds majority required. At the last minute many of the liberals attempted to rally around 78-year-old Cardinal Martini, the former Archbishop of Milan, but it was too late. Martini pleaded with them to vote for Ratzinger for the sake of the unity of the church.

So how did the cardinals come to choose an ageing, deeply conservative, ecclesiastical bureaucrat, known within his church — not just in the British press — as “Enforcer”, “Rottweiler”, “Panzer Kardinal”? The answer lies within the close relationship between Ratzinger and John Paul II, the most dominant pope in history, and the powerful single-minded conservative constituency he has created within the church through 26 years.

Joseph Aloysius Ratzinger, the son of a policeman, was born on April 16, 1927, in Marktl am Inn in Upper Bavaria, a Roman Catholic stronghold. He felt an early call to the priesthood and, at the age of 11, went to the seminary in nearby Traunstein.