ERFURT, Germany (Reuters) - Pope Benedict, visiting the German monastery where Martin Luther lived before launching the Reformation, warned his Lutheran hosts on Friday that what he called "a new form of Christianity" posed a challenge to mainline Protestants and Catholics alike.
While not naming them, it was clear that the pope, whose visit to this small city south of Berlin was sparsely attended, was referring to the evangelical and Pentecostal churches which have been attracting converts from more established churches, especially in Third World countries.
"Faced with a new form of Christianity, which is spreading with overpowering missionary dynamism, sometimes in frightening ways, the mainstream Christian denominations often seem at a loss," the pope said on the second day of his third trip to his homeland as pontiff.
"This is a form of Christianity with little institutional depth, little rationality and even less dogmatic content, and with little stability. This worldwide phenomenon poses a question to us all: what is this new form of Christianity saying to us, for better and for worse?"
The pope's visit to Germany has come at a time when record numbers of the faithful have quit the pews in the past year, in part to protest against clerical sex abuse of youths. About 181,000 disenchanted Catholics left the German Church in 2010.
Benedict appealed for unity between Roman Catholics and Protestants, who began their split from the church in the 16th century with the posting by Luther, who lived in Erfurt as a Catholic monk, of his 95 Theses in 1517.
At the same time, he deflected appeals from Protestants for a relaxation of rules barring them from participating in Catholic communion.
He said it was a "political misunderstanding" to think he would come with an "ecumenical gift" such as permission for Protestants to receive the Catholic eucharist.
Germany's top Protestant bishop urged the pontiff to take "real steps for reconciliation" in advance of the upcoming 500th anniversary of the Reformation in 2017, which the Protestants would like to mark with Catholic participation.
Luther is "as a hinge between our two churches, because he belongs to both," said Lutheran Bishop Nikolaus Schneider, head of the Evangelical Church in Germany (EKD. "It is time to heal the memories of the mutual injuries in the Reformation period and the subsequent history of our Churches."
Chancellor Angela Merkel, daughter of a Lutheran pastor, attended the service highlighted by interludes of sacred organ music by Johann Sebastian Bach and other German composers.
But in other respects Erfurt, in a traditionally Lutheran and atheist region, gave the pope a cool reception. There were few posters or banners announcing his presence. Strict security measures in central Erfurt may have also kept the curious away.
Two pensioners who travelled from mostly Catholic Bavaria to see the pope were surprised at the thin turnout. "It's completely empty here, perhaps because this is former East Germany so there are more atheists," said Anton Messer.
In a shop near the cathedral, atheist Dagmar Schneidert said: "I don't know anyone who is enthusiastic about the visit, only 7 percent of people in Erfurt are Catholic anyway. It's wasted money that could be used for something more meaningful."
The pope is due to hold an open-air Mass in central Erfurt on Saturday that Church officials expect 85,000 faithful.
Before leaving Berlin, Benedict met leaders of Germany's four million Muslims and pledged cooperation with them as long as they respect Germany's constitution and the limits it sets on pluralism. Angela Merkel warned Muslims last year that sharia, Islamic law, had no place in Germany.
Muslim leaders praised the pope for confirming through the meeting that Islam was now a part of German society. But they said their loyalty to the constitution was never in question.
"As Muslims in Germany, we have always said that we see the German constitution as a good basis for peaceful life together," Bekir Alboga, head of interreligious dialogue for the Turkish mosque association DITIB, told Reuters after meeting the pope.
Aiman Mazyek, chairman of the Central Council of Muslims in Germany, told reporters: "My impression was that the pope wants to launch a new era of dialogue with Muslims."
Alboga said the pope's short address also represented a change from his controversial 2006 speech in Regensburg, where his use of a Byzantine emperor's quote about Islam being violent and irrational sparked heated protests across the Muslim world.
"The pope has now chosen a new approach in his meeting with Muslims," he said. "I think one must look to the future and see where the possibilities for good cooperation are."
The Bavarian-born pontiff began his third and most challenging visit to his homeland on Thursday with an address to parliament and an open-air Mass at Berlin's Olympic Stadium.
About 8,000 people protested in central Berlin against his conservative moral teachings and the clerical sexual abuse scandals shaking the Church. Almost 100 left-wing deputies boycotted his speech to the Bundestag lower house of parliament.
(Writing by Philip Pullella and Tom Heneghan; Editing by Michael Roddy)