AS the threat from Isis grows starker, the caliphate's intellectual underpinnings have become clear: the missionary efforts of Wahhabi preachers, fuelled by tens of billions of dollars of Saudi oil money, that have saturated the Muslim world in the past 40 years.
This is the rigid, medieval version of Islam which decrees that infidels be killed and their womenfolk violated - that such behaviour is the devout Muslim's religious duty.
Millions of Muslims, praying in the ugly mosques built with Saudi money, have absorbed this hate-ridden version of the religion with their mothers' milk.
Abu Bakr al-Baghdadi and his followers are piously putting it into practice, with the results that we see.
It is not only the Shia, the Christians and the Yazidis who are suffering in consequence; the Sufis of Afghanistan and Kashmir and the Muslims of northern India, whose religious practices are intertwined with Hinduism, have been forced into silence.
Great artists like the late Nusrat Fateh Ali Khan, the legendary qawwali singer of Pakistan, are condemned as un-Islamic by imams for whom all music is the Devil's work.
Against this background of atrocity and intolerance, a very large Muslim organisation in the world's most populous Muslim country this week declared that the time has come to fight back.
Nahdlatul Ulama, an Indonesian Muslim organisation, claims to have 50 million members, more than one in four of Indonesia's Muslims.
This week it released a feature-length film, The Divine Grace of East Indies Islam, which, from a foundation of Sunni theological orthodoxy, challenges Isis head on.
Against Isis footage of prisoners being marched to a river bank, shot in the head and dumped in the water, the soundtrack declares, ''Many who memorise the Koran... love to condemn others as infidels while ignoring their own infidelity to God, their hearts and minds mired in filth.''
''We are directly challenging the idea of Isis, which wants Islam to be uniform, meaning that if there is any other idea of Islam that is not following their ideas, those people are infidels who must be killed,'' Yahya Cholil, general secretary of the organisation's supreme council, told the New York Times. ''We will show that is not the case with Islam.''
Like European countries with large Muslim populations, Indonesia has suffered its share of terrorism from the Islamofascists, most notoriously the Bali bombings of 2005 (180 dead) and 2015 (seven dead), which were aimed at foreign tourists.
Non-Sunni Muslims and minority religions have also been targeted, with churches forced to close, Shia Muslims attacked, and Ahmadiyya Muslims, stigmatised as heretics, harassed and intimidated.
In the province of Aceh, where coastal areas were destroyed in the 2004 tsunami, with the loss of more than 230,000 lives, Saudi-style Islamism has taken root.
Fundamentalists interpreted the tsunami as God's punishment for sinful behaviour, and the imposition of Sharia law followed, with corporal punishment for gambling and drinking alcohol and women obliged to wear headscarves in public and other repressive measures. But Aceh has always been exceptional: as the spot where Islam arrived in South East Asia in the 15th century, it has always been the most religiously conservative part of the country.
The rest of the country continues to hold out against such policies.
The main island of Java was a major centre of both Hinduism and Buddhism for more than a millennium before Islam swept all before it, and the Indonesians continue to take pride in the richness of their cultural heritage: the extraordinary ninth century Buddhist complex at Borobudur, near Yogyakarta, survives, despite bombs in the 1980s and recent threats by Isis to destroy it, and continues to stage performances of the Ramayana, the Hindu epic.
''Islam,'' VS Naipaul wrote in 1981 in his book Among the Believers, ''which had come only in the 15th century, was the formal faith.
But the Hindu-Buddhist past, that had lasted for 1,400 years before that, survived in many ways - half erased, slightly mysterious, but still awesome, like Borobudur itself. And it was this past which gave Indonesians... the feeling of their uniqueness.''
The rich, even baffling diversity of Indonesia's past is matched by the complexity of its present, and makes it more comfortable in a globalised world than countries like Saudi Arabia, glued to a one-dimensional tradition of zealotry.
In 2009, on her maiden overseas trip as secretary of state, Hillary Clinton told her hosts: ''As I travel around the world... I will be saying to people, if you want to know whether Islam, democracy, modernity and women's rights can co-exist, go to Indonesia.''
It has held fast to the principles of the Non-Aligned Movement, keeping out of exclusive alliances and maintaining good relations with countries as diverse as Russia, China and Japan, as well as the United States and Saudi Arabia.
It is no accident that Nahdlatul Ulama has come out fighting against Isis this week: the organisation was founded in 1926 in reaction to the spread of Wahhabbism, and has defined itself in opposition to the fundamentalists ever since.
The spiritual leader of the group, Ahmad Mustofa Bisri, said: ''Highly vocal elements within the Muslim population... justify their harsh and often savage behaviour by claiming to act in accord with God's commands.''
They are, he said, ''grievously mistaken.''
Can Muslims so far from Arabia have any influence on opinion in the religion's heartland? They stand a better chance than the likes of Messrs Cameron and Hollande.
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