The clerical abuse scandal has plunged the Catholic Church into a crisis unlike anything it has experienced since the Protestant Reformation half a millennium ago. Pope Benedict, the Curia (the ecclesiastical government of the Church in Rome) and the Catholic bishops of the world have been at pains to minimise the appalling PR. But the Pope's attempts to understand the underlying causes of clerical abuse have been piecemeal and inchoate. Against this background, any positive contribution towards an understanding of the underlying reasons for the calamity must be welcomed by Catholics and non-Catholic Christians alike - for a failing Catholic Church is a failing Christendom.

Geoffrey Robertson's scalding j'accuse against priestly paedophile perpetrators, the Vatican and the current Pope will likely infuriate most devout Catholics, especially as its publication is timed to coincide with a papal state visit celebrating Benedict's spiritual leadership of his British flock. Written in a series of numbered paragraphs, like lawyer's briefs, the book attempts to address the circumstances which, in Robertson's view, have enabled the Pope to evade justice for his alleged role in covering up the abuse scandal.

He indicts Benedict personally, both as Pope since 2005 and in his former role as head of the Congregation for the Doctrine of the Faith from 1981, for failing to turn paedophile priests over to the civil authorities. Robertson, a senior barrister specialising in human rights cases, moreover argues that the Vatican's pretensions to sovereign statehood have granted the Pope immunity from standing trial for his part in the cover-ups. He also reasons that canon law, the system of Church law which is independent of the civil and criminal laws of authentic nation states, makes prelates and priests complicit in these sex crimes by swearing them to secrecy about any knowledge they might have of clerical abuse. Robertson calls on the civilised world to reject the Pope's title as head of state and to outlaw the Catholic Code of Canon Law.

By any measure, the statehood of the Vatican is an anomaly. The author derides its size - it is "smaller than Disneyland" - and its provenance as an element of a deal to prop up fascism in 1929. He goes on to argue that the papacy has exploited this sovereign statehood by routinely exerting an unwarranted influence in the forums of proper nation states on questions such as abortion, contraception and women's rights. Robertson asserts that, by the same token, the community of nations has failed to treat the Vatican as an erring state by declining to denounce the way it protects sexual abusers. He notes: "The UN's ineffectual 'committee of experts', which pretends to supervise compliance with the convention [on the rights of the child], has said not a word."

Being a case for the prosecution, the book refrains from offering even minimal acknowledgment of a potential defence. A Catholic response to these views on Vatican statehood would invoke not only the origins of the papal states dating back to the Middle Ages, but also the historic trauma experienced by the papacy as a result of military sequestration by Italy of papal territories between 1850 and 1870. These states, running from just north of Naples virtually up to the Veneto, constituted a sovereign, independent European monarchy. The Lateran Pacts of 1929, which gave popes the 109-acre territory that the Holy See now calls its own, were agreed in recognition of the papacy having been deprived by force of its financial, national and political independence.

Whatever Mussolini's motives for signing that agreement, Catholics recognised that if the pope does not have a sovereign territory, then he becomes either stateless or the citizen of another nation state, and so subject to its laws - which might be inimical to Catholic doctrine and an impossible position for the leader of a worldwide Church to find himself in. Catholics would argue that despite Pius XII's failure to speak out strongly against the Nazi Holocaust, his tiny territory, surrounded by, but independent of, the Italian fascist state, enabled him to continue his leadership of the universal Church through the war years.

By the same token, the Code of Canon Law was devised not to create a conspiracy of silence that would protect erring priests (that was a
by-product), but to bring order to a jungle of Church case law. Canon law sets out many hundreds of relationships between the clergy and laity on matters as diverse as the dedication of churches, the process of annulments and the nomination of bishops. Its need to run a unified institution in every quarter of the globe is obvious. To call for its abolition is unrealistic. To call for reforms to specific rules relating to abusing priests, however, makes sense.

Where Robertson's "case" gathers force is in his critique of Catholic clericalism. He acknowledges that the priesthood attracts many good and decent men. However, unearned respect for the celibate caste attracts men who are "psychosexually immature, often in denial about their condition and hoping that the rigours of the priesthood will protect them from themselves".

He goes on: "Instead, they find a brotherhood, a solidarity that closes ranks to protect them not from themselves but from the con­sequences of their actions, because the overriding philosophy of their superiors has been to avoid scandal to the Church." He is also right to claim that ready forgiveness in confession binds clerics and confessors in a conspiracy where secret sexual transgressions are minimised and trivialised.

Unfortunately, Robertson has a tendency to beef up his indictment by invoking dubious extraneous charges that will inevitably be cited to dismiss his main argument. The shaky allegation that John Paul I was poisoned in the Vatican in 1978, and that the guilty ones evaded justice because Italian law could not order an autopsy, does his case no favours. John Paul I was not healthy, as Robertson admits. He was being treated for blood coagulation and embolism with drugs that he failed to take. The morticians were not called at 4am, before the pope's dead body was found that morning, as Robertson alleges, but - as I discovered when interviewing them before witnesses in 1988 - at 4pm the same day.

Nor is it true that the Vatican Bank evaded responsibility for the Banco Ambrosiano scandal: in 1984, the Holy See paid a quarter of a billion dollars in compensation and fines to the Italian government. As for the accusation that the papacy sanctioned a Nazi rat run, a team of distinguished journalists, including Magnus Linklater and Neal Ascherson, attempted over a period of six months during the 1980s to discover evidence of papal involvement. They found none.

The bare facts of the paedophile priest scandal, and the attempts to cover them up, are self-evident and appalling enough without embellishment. The Church urgently needs to examine the systemic basis of a clerical corruption that Benedict himself has come to characterise, belatedly, as "filth". I hope that Catholic officialdom will take note of Robertson's more salient points. But I fear that those most in need of giving his case a hearing will be those least disposed to listen.

John Cornwell's "Newman's Unquiet Grave" is published by Continuum (£18.99)