After trying in vain for decades to convince his own church to liberalise, Roman Catholicism's leading dissident theologian, Hans Kueng, is turning his attention towards reforming another world faith - Islam. Tom Heneghan reports from Paris.
His latest book, Islam, which goes on sale in Germany next week, is unlikely to meet a better reception among Muslims than his ideas have had from conservative critics in the Vatican.
Over 891 pages, the blunt Swiss theologian describes Islam as intellectually outdated, biased against women, intolerant of other religions and hostile to key human rights. But unlike many Westerners, he also offers a sympathetic analysis of Islam's evolution compared to the way Christianity and Judaism have adapted to the modern world, voicing appreciation of its focus on faith and economic justice.
And fewer still offer detailed proposals - what he calls a "best-case scenario" - for turning Islam from a perceived enemy of the West to a bridge of understanding among cultures.
"I'm writing as a Christian, so they can't expect me to agree with everything," Kueng said in a telephone interview from the German university town of Tuebingen.
"I'll have to see how it's received. Anyway, this is a long-term project," said the 76-year-old priest, who was banned by the Vatican from teaching Catholic theology in 1979 for challenging papal infallibility.
Kueng, who urges Catholicism to ordain women and move closer to other Christian churches, not surprisingly comes out for reforms advocated mostly by liberal Muslim thinkers in Europe.
Sharia law, the hallmark of any traditionalist or fundamentalist Islamic state, violates universal human rights with its chopping off of hands for thieves and flogging or death penalties for adulterers and apostates, he writes.
"How long can conservative Muslims continue to say these gruesome punishments are the retribution of a just God?"
The Koran, which Muslims revere as the literal word of God revealed to the Prophet Mohammed as well as a guide for daily life, cannot be read "as a system of fixed formulas, rigid doctrines and unchangeable laws", Kueng says.
Instead, it should be a "living message that must always be seen anew". That would take the ideological edge off arguments Muslims use to defend headscarves, especially in Western countries where some schoolgirls insist on them as a right and religious duty even though the Koran does not prescribe it, he says.
"Headscarf activists in Europe are not convincing when they demand their human rights but stay silent about the inhuman treatment of women in traditional Muslim countries," he writes.
Kueng appreciates Islam's focus on restrained sexual morals at a time when Western societies know no taboos. He also praises its concern for economic justice, but dismisses the Koran's ban on interest as an obsolete idea.
He calls for an open theological interpretation of such rules, but says no consensus among Muslims is in sight.
Despite these hurdles, Kueng, whose Global Ethic Foundation promotes universal ethical norms among world religions, is as ambitious for Islam as he is for his own church.
"If today even the Catholic Church, which was for so long a bulwark of anti-democratic reaction, at least theoretically supports human rights, can't Islam also make a fundamental change?"
Now only available in German, the book is expected to be published in English, French, Italian and Spanish over the next year or so.
"I hope it will appear in Arabic," the undaunted rebel said.