Bishops who have been meeting for three weeks to plot a way forward for a Church whose membership is dwindling in Europe are concerned by Islam's growth and worried about Christian minorities in Muslim countries, according to participants' comments released by the Vatican.
Islam was barely mentioned in preparatory documents for the Synod on New Evangelisation, a meeting in Rome of 262 prelates from around the world been held behind closed doors.
But one participant said it had become the "buzzword" of the synod that ends this weekend.
"It's no surprise that Islam has taken on such importance during this synod," French-born Bishop Paul Desfarges, who heads the diocese of Constantine in Algeria, told journalists in Rome this week. "It's an issue that concerns Europe."
Christianity, with about 2 billion followers, is the world's largest religion and Catholicism - its biggest denomination by far - makes up just over half that total.
But some estimates suggest that the 1.3 billion Muslims, four-fifths of them outside the Arab world, are growing in number much faster than Christians, whose numbers are shrinking in their European heartland.
"We need a much more developed analysis and discussion of the consequences of the Islamic presence in the Western world," Sydney Cardinal George Pell said.
POLITICAL BOOST FOR ISLAM
Some of the "Arab Spring" uprisings that have spread across North Africa and the Middle East have propelled Islam onto the political stage.
Kyrillos William, the Catholic Coptic bishop of Assiut, painted a stark picture of the situation facing Egypt's large Christian minority - about 10 percent of the population - since the upheavals of the Arab Spring.
"Every day since the Muslim Brotherhood's rise to power, we see new steps towards the Islamisation of the state," he said. "Christians continue to be considered second-class citizens and many of their rights are not recognized."
In a remark that Church leaders interpreted as criticism, a senior Egyptian official has said the Church's many schools and hospitals, which are used mostly by Muslims, gave it a presence in society much bigger than its actual size.
"Some extremists demand that we leave the country," the bishop said. "We've told them: 'No, this is our country and we're staying'."
In West Africa, where Christianity and Islam are vying for new followers among the many people quitting traditional religions, bishops felt Catholicism had a double disadvantage.
"The rapid expansion of Islam and especially the spreading of fundamentalism in West Africa enormously worries the Church," said Bishop Nicodeme Anani Barrigah-Benissan from Togo.
"It only takes one day to become Muslim but it is impossible to renounce this religion later," he said. By contrast, he added, it takes at least three years of study for an adult to become a Catholic, and the baptized can leave at will.
Patriarch Gregory III Laham, the Damascus-based head of the Melkite Greek Catholic Church, argued that Islam's main teaching - that there is no God but Allah and Mohammad is his prophet - was easier for people to grasp.
"Our beautiful Christian faith is too complicated," he said.
CONVERSION ILLEGAL
Conversion from Islam to another religion is illegal in many Muslim countries, meaning that Christians dedicated to spreading their faith must do so very cautiously, several prelates noted.
Bishop Desfarges reported that some Algerian Muslims had converted to Catholicism.
"These new disciples are sometimes rejected by their own family or must be very discreet," he said.
Bechara Boutros Rai, the Beirut-based head of the Maronite Church in communion with the Vatican, said there were also "some secret conversions by Muslims to Christianity" in Lebanon.
"Evangelisation is practiced in the Arab countries in an indirect way," he said, through a wide network of Christian schools, hospitals and broadcast media that spread the Church's message without explicitly preaching it.
But several bishops, especially from countries where tensions between Christians and Muslims seem to be worst, saw the meeting of the two faiths as an opportunity for dialogue rather than a reason to bemoan their fate.
"Despite the impression often given by the world media, I want to stress that Christians in Nigeria do not see themselves as being under any massive persecution by Muslims," Abuja Archbishop John Olorunfemi Onaiyekan told the synod.
"We need to know our Muslim neighbors and keep an open mind to those who are friendly, and they are in the majority," he said. "We have to work together to make sure that the fanatics do not dictate the agenda of our mutual relations."
The Islam debate at the conference got off to a rocky start when Cardinal Peter Turkson, head of the Vatican Council for Justice and Peace, screened a video suggesting that Europe was quickly being overrun by Muslims.
Cardinal Andre Vingt-Trois from France, home to Europe's largest Muslim minority, panned the film as propaganda and said the New Evangelisation "should not become a Crusade".