Sexual abuse of children is not unique to the Roman Catholic church, and Joseph Ratzinger is not one of those priests who raped altar boys while in a position of dominance and trust. But as so often it is the subsequent cover-ups, even more than the original crimes, that do most to discredit an institution, and here the pope is in real trouble.

Pope Benedict XVI is the head of the institution as a whole, but we can't blame the present head for what was done before his watch. Except that in his particular case, as archbishop of Munich and as Cardinal Ratzinger, head of the Congregation for the Doctrine of the Faith (what used to be called the Inquisition), the very least you can say is that there is a case for him to answer. See, for example, three articles by my colleague Christopher Hitchens here, here, and here. The latest smoking gun is the 1985 letter obtained by the Associated Press, signed by the then Cardinal Ratzinger to the diocese of Oakland about the case of Father Stephen Kiesle, mercilessly analysed by Andrew Sullivan here.

Lashing out in desperation, church spokesmen are now blaming everybody but themselves for their current dire plight, which one official spokesman likens to the worst aspects of antisemitism (what are the best ones, I wonder?). Suggested culprits include the media, the Jews, and even Satan. The church is hiding behind a seemingly endless stream of excuses for having failed in its legal and moral obligation to report serious crimes to the appropriate civil authorities. But it was Cardinal Ratzinger's official responsibility to determine the church's response to allegations of child sex abuse, and his letter in the Kiesle case makes the real motivation devastatingly explicit. Here are his actual words, translated from the Latin in the AP report:

"This court, although it regards the arguments presented in favour of removal in this case to be of grave significance, nevertheless deems it necessary to consider the good of the universal church together with that of the petitioner, and it is also unable to make light of the detriment that granting the dispensation can provoke with the community of Christ's faithful, particularly regarding the young age of the petitioner."

"The young age of the petitioner" refers to Kiesle, then aged 38, not the age of any of the boys he tied up and raped (11 and 13). It is completely clear that, together with a nod to the welfare of the "young" priest, Ratzinger's primary concern, and the reason he refused to unfrock Kiesle (who went on to re-offend) was "the good of the universal church".

This pattern of putting church PR over and above the welfare of the children in its care (and what an understatement that is) is repeated over and over again in the cover-ups that are now coming to light, all over the world. And Ratzinger himself expressed it with damning clarity in this smoking gun letter.

In this case he was refusing the strong request of the local bishop that Kiesle should be unfrocked. Vatican standing orders were to refer such cases not to the civil authorities but to the church itself. The current campaign to call the church to account can take credit for the fact that this standing order has just changed, as of Monday 12 April 2010. Better late than never, as Galileo might have remarked in 1979, when the Vatican finally got around to a posthumous pardon.

Suppose the British secretary of state for schools received, from a local education authority, a reliable report of a teacher tying up his pupils and raping them. Imagine that, instead of turning the matter over to the police, he had simply moved the offender from school to school, where he repeatedly raped other children. That would be bad enough. But now suppose that he justified his decision in terms such as these:

"Although I regard the arguments in favour of prosecution, presented by the local education authority, as of grave significance, I nevertheless deem it necessary to consider the good of the government and the party, together with that of the offending teacher. And I am also unable to make light of the detriment that prosecuting the offender can provoke among voters, particularly regarding the young age of the offender."

The analogy breaks down, only in that we aren't talking about a single offending priest, but many thousands, all over the world.

Why is the church allowed to get away with it, when any government minister who was caught writing such a letter would immediately have to resign in ignominy, and face prosecution himself? A religious leader, such as the pope, should be no different. That is why, along with Christopher Hitchens, I am supporting the current investigation of the pope's criminal complicity by Geoffrey Robertson QC and Mark Stephens. These excellent lawyers believe that, for a start, they have a persuasive case against the Vatican's status as a sovereign state, on the basis that it was just an ad hoc concoction driven by internal Italian politics under Mussolini, and was never given full status at the UN. If they succeed in this initial argument, the pope could not claim diplomatic immunity as a head of state, and could be arrested if he steps on British soil.

Why is anyone surprised, much less shocked, when Christopher Hitchens and I call for the prosecution of the pope, if he goes ahead with his proposed visit to Britain? The only strange thing about our proposal is that it had to come from us: where have the world's governments been all this time? Where is their moral fibre? Where is their commitment to treating everyone equally under the law? The UK government, far from standing up for justice for the innocent victims of the Roman Catholic church, is preparing to welcome this grotesquely tainted man on an official visit to the UK so that he can "dispense moral guidance". Read that again: dispense moral guidance!

Unfortunately I must end in bathos, with a necessary correction of a damaging error in another newspaper. The Sunday Times of 11 April, on its front page, printed the headline, "Richard Dawkins: I will arrest Pope Benedict XVI." This conjures up – as was doubtless intended – a ludicrous image of me ambushing the pontiff with a pair of handcuffs and marching him off in a half Nelson. Blood out of a stone, but I finally managed to persuade that Murdoch paper to change the headline in the online edition.

Never mind headlines invented by foolish sub-editors, we are serious. It should be for a court to decide – a civil court, not a whitewashing ecclesiastical court – whether the case against Ratzinger is as damning as it looks. If he is innocent, let him have the opportunity to demonstrate it in court. If he is guilty, let him face justice. Just like anybody else.