Pretoria, South Africa - Winding its way across Africa is an invisible faultline between a mainly Muslim north and a majority Christian south.
Across this front, somewhere in the scrubland south of the Sahara desert, tensions flare between increasingly strident Christian movements aggressively working to convert people of other faiths and an Islamic world wary of encroaching Western influences.
When the two worlds collide - a Muslim takes offence at a woman showing too much skin or a shop selling alcohol, or a Christian takes umbrage when a thief's hand is chopped off - entire villages can riot Africa's 1-billion people offer the largest field of potential converts anywhere in the world and the competition for souls is fiercest between the continent's two biggest religions - Catholicism and Islam.
"The Vatican recognises that in sub-Saharan Africa you have the area where Muslims and Christians are most likely to be confronting each other," said John Voll, director of the Centre for Muslim-Christian Understanding at Georgetown University in Washington DC.
"It is a high priority to make sure that those Catholic interactions with Muslims don't necessarily lead to open conflict."
But the ancient battle has exploded across an invisible frontier separating not just religions but also cultures and major ethnic groups, and ancient divisions between herders and settled farmers.
The 21-year civil war that recently ended in Sudan erupted when the Arab government imposed strict Islamic law opposed by blacks in the south, where more than 4-million Catholics make up 13% of the population. That war is blamed for more than 2-million deaths.
Tens of thousand of others have been killed across the religious faultline in conflicts in Liberia, Ivory Coast and Nigeria, whose estimated 20-million Catholics are outnumbered on the continent only by the Congo's 28-million-plus adherents.
The decision of British colonisers to rule northern Nigeria through Muslim emirs while Irish Catholic missionaries proselytised in the south has produced a nation where tribal divisions are transcended only by religion.
Riots erupted in the country in 2000, when the mainly Muslim northern states instituted Islamic law, including punishments of amputation and death by stoning.
In 2002, more than 200 people died in Christian-Muslim riots triggered by opposition to holding the Miss World competition in Nigeria.
This month, Nigerian newspapers have been full of reports that Islamic leaders are preparing a violent jihad, which they deny.
Islamic leaders, meanwhile, complain Muslims are being marginalised under a southern Christian president who ended 20 years of northern rule.
Pope John Paul II chose Nigeria's most powerful cleric, Cardinal Francis Arinze, to lead the church's efforts to build relations with other religions at a time when fundamentalist Islamic and Protestant sects replaced communism as the biggest challenge to Catholic proselytising.
Arinze, whom some consider a top contender to succeed John Paul II, took the route of stressing Islam and Catholicism's common fight against sexual permissiveness and contraception.
"Authentic dialogue demands that Muslims and Christians accept one another with all their similarities and differences," he told students at the Roman Catholic Georgetown University in 1993.
He said: "Muslim-Christian relations are challenged and obstructed by religious fanaticism or extremism."
The more extreme forms of Christianity and Islam are gaining strength in Africa, increasing the risk of more confrontation, especially as the two sides compete for converts from each other's camps as the ranks of followers of traditional animist religions shrink.
Reverend Matthew Hassan Kukah, an influential Nigerian Catholic priest, reserved his most trenchant criticism for evangelical Christians rather than for Muslims, saying they are promoting a materialist view of Christianity and stoking tensions by behaving "as if they were out to convert everyone".
Muslims have rioted when evangelical churches hold football-stadium gatherings.
"There is too much aggressive evangelisation," said Latif Adegbite, secretary-general of Nigeria's National Supreme Council for Islamic Affairs.
But Islam is also on the march, and today the religious faultline on the continent is shifting south.
"The Catholic Church has become aware that Islam has become a serious alternative" in Central and West Africa, said Voll. To face competition from Islam, he said, the church would need to "reconstruct itself as a refuge for people in times of social crisis".
The Catholic Church failed that test during the genocide in Rwanda, where some churches that offered sanctuary were turned into slaughterhouses and some Catholic priests actively participated in the killings.
John Paul II refused to apologise, saying the church could not be blamed for the actions of individual priests.
Islam offers a route more compatible with African tradition, especially by allowing polygamy. Some Catholic Africans marry their first wife in the church, then take second wives in
traditional ceremonies.