For many years now, and in different versions, a joke about German Cardinal Joseph Ratzinger has been doing the Vatican rounds. The action takes place outside the gates of Heaven, where three new would-be residents have just arrived, Brazilian liberation theologian Leonardo Boff, dissident German theologian Hans Kung and Cardinal Ratzinger.

St Peter emerges from his shed-cum-gate-lodge to assess the new arrivals, telling them he wishes to interview all three. Leonardo Boff is the first to be shown into St Peter's office.

Four hour later an exhausted-looking Boff emerges and stumbles over to the Pearly Gates, which open for him, muttering as he goes: "Oh, my goodness, that was so tough . . . I just never knew, I'm so sorry."

Hans Kung is the next man into St Peter's office. His stay is even longer, and after six hours he emerges, looking distraught and crushed. He, too, is muttering to himself: "Oh, my goodness, that was so tough . . . I just never knew, I'm so sorry, how could I have been so wrong."

Finally, it is Ratzinger's turn. His session with St Peter turns out to be longest of the lot.

After 72 hours, however, it is the exhausted, distraught St Peter himself who emerges from his office, heading for the Gates of Heaven and muttering to himself: "Oh, my goodness, that was so tough . . . That was the hardest thing I've ever done . . . How could I have been so wrong."

Cardinal Joseph Ratzinger, Prefect of the Vatican's Congregation for the Doctrine of the Faith, has for long been known as the Holy See's Watchdog of Orthodoxy, the "latter-day Inquisitor General", the German Panzerkardinal who, ogre-like, will annihilate any and every form of theological dissidence within the Roman Catholic Church.

Ask those not intimately familiar with the Vatican to name a senior Curia figure, and probably the only cardinal's name that will come to mind is that of Joseph Ratzinger.

Apart, obviously, from Pope John Paul II, he is internationally the best-known Vatican figure, certainly much better known than Curia makers and shakers such as "Prime Minister" (Secretary of State) Cardinal Angelo Sodano or "Home Office" minister (Substitute for General Affairs) Archbishop Giovanni Battista Re.

For some, Cardinal Ratzinger's department, the Congregation for the Doctrine of the Faith (formerly Holy Office), has remained all too true to its original brief when founded by Pope Paul III in 1542 as the Sacred Congregation of the Universal Inquisition, called on to combat all forms of heresy, real or imagined. For American Catholic and religious affairs writer George Weigel, on the other hand, Cardinal Ratzinger is someone gifted with "a great mind and a great Christian spirit" (The Catholic Northwest Progress, 1997).

Discreet and modest in his private life, Cardinal Ratzinger lives just off St Peter's Square, close to the Porta Santa Anna, in an apartment shared, until her death in 1991, with his sister, Maria. It is not unusual to see him emerge from his flat, close to what used to be the No 64 bus stop familiar to millions of Vatican tourists, togged out in full regalia and about to walk across the square to attend a Vatican function. No police escorts and no flashing lights signal his presence.

On the day last month when he showed up at the Vatican Press Office to present the controversial declaration known as Dominus Iesus he looked, as he always does on such occasions, more like a retiring, silver-haired and unworldly academic than a zealous tyrant about to throw latter-day Giordano Brunos or Galileo Galileis on to the pyre.

Vatican insiders confirm this impression, pointing out that he largely avoids the Vatican social circuit of embassy receptions, book launches and conferences, limiting his public speaking to three or four seminars per annum, notwithstanding a myriad invitations from all over the world.

German friends also point out that he has only one close friend in the Vatican, retired German Cardinal Paul Mayer who, like him, comes from Bavaria. Cardinal Mayer, along with his much-loved brother, Georg, with whom he spends his holidays in Bavaria, are his closest friends.

The storm over his implied questioning in Dominus Iesus of the legitimacy of Protestant churches came as no surprise to German Catholics, who have watched with discomfort as he transformed himself from a young, liberal theologian into an uncompromising guardian of the orthodox.

Born in the Bavarian town of Marktlam Inn in 1927, Rat zinger was the son of a devout Catholic policeman who opposed the Nazis from the beginning of Hitler's rise to power. Despite his poor background,

Ratzinger never cultivated the common touch, and from the moment he was ordained a priest in 1951 it was clear that his career would be dominated by theological study rather than pastoral work.

He first came to public attention when he was appointed adviser to the Archbishop of Cologne, Cardinal Joseph Frings, at the Second Vatican Council in 1962.

Then just 35 years old, Ratzinger was an enthusiastic supporter of the council's mission to make the church more open and to admit to its institutions the fresh air of new thinking.

But it was the student revolt of 1968 that caused him to change course, prompting him to cling to the certainties of traditional church teaching as he watched what he perceived to be a violent attempt to overthrow the existing order. Appointed Archbishop of Munich in 1977 and made a cardinal a few months later, he was among a handful of German prelates who shared the conservative vision of John Paul II.

German Catholics, most of whom are liberal in matters of faith and keen supporters of closer links with other churches, have had little to celebrate during Ratzinger's years at the Vatican. When a group of lay Catholics called Wir Sind Kirche (We Are Church) organised a petition two years ago calling for a more open, democratic church, Ratzinger was openly contemptuous of their efforts.

"An excess of papers, of discussions, of pointless organisations in which there is a danger of flogging the faith to death. And this displays a certain German provincialism in which one is convinced that one knows everything best," he said.

Despite this putdown, German friends describe Ratzinger as warm, kind and with a surprising sense of humour, in sharp contrast to the serious nature of his Vatican office. One German religious affairs reporter recalls the day when she found herself sitting beside him on a Rome-Munich flight. She had been trying to talk to the cardinal about a Vatican issue during the week, but had kept on missing him.

As Cardinal Ratzinger looked up and realised who his companion for the journey would be, he sighed deeply and said: "Now, this is Divine Providence."

Inside the cosy, academic exterior, however, beats a rigidly orthodox heart. His Congregation's brief, as defined at Vatican Council II, suggests that "the defence of the faith today is better provided for by encouraging good theology". In his interpretation of that brief, however, Cardinal Ratzinger may occasionally have substituted the adjective orthodox for good, in relation to theology.

Certainly, many Catholics were disturbed by his handling in the early 1980s of dissident or liberation theologians such as Brazil's Boff, the Peruvian Gustavo Gutierrez, the Belgian Edward Schillebeeckx and the German Hans Kung, all of whom were severely reprimanded by the Congregation for the Doctrine of the Faith.

In the case of Boff, who was effectively silenced, the experience was to prove traumatic, and he has since renounced the priesthood. In the case of others such as Kung, the term silenced can hardly be applied, since Kung's dissident writings have in the meantime made him a regular, religious best-seller.

If, in the 1980s, Cardinal Ratzinger's bete noir appeared to be liberation theology, his attention in the 1990s has switched to "religious relativism". Since the collapse of Marxism in the late 1980s, the emergence of a pluralist theology - saying basically that one religion is as good as another - has increasingly preoccupied Cardinal Rat zinger, as evidenced most recently with Do- minus Iesus.

In a 1997 book-cum-interview, Salt of the Earth, the cardinal suggested that liturgy "lives from what is unmanipulable". Catholic liturgy cannot be equated to car design and its task is not to come up with new models every year, he added.

Commentators have been tempted to suggest that Cardinal Ratzinger has been setting down markers with a view to the agenda for the next pontificate, taking advantage of the Pope's all too obvious physical frailty to seize the tiller of the Catholic Church and steer it into traditionalist, conservative waters.

Senior Curia figures categorically reject this interpretation, pointing out not only that the Pope remains mentally alive and well, but also underlining the fact that John Paul II himself chose to emphasise the importance of Dominus Iesus last Sunday during his Angelus address, describing it as "a document approved by me" but one which contains "no arrogance towards, or disrespect for, other religions".

Cardinal Ratzinger is no maverick ploughing a lone furrow. On the contrary, he is the living expression of the theologically orthodox, rigidly dogmatic nature of this pontificate, an orthodoxy that has often been overshadowed by the charismatic evangelising of this globe-trotting Pope.