A pastor, a rabbi and an imam in Berlin hope to create history by bringing together people of different faiths and praying under the same roof.
Still a sand-strewn vacant construction site, St Peter's Square in the centre of the German capital will -- God willing -- by 2018 host a building that's so unusual it doesn't have an official term.
Not a church, nor a synagogue, or a mosque as such, but a bit of all three, the centre known currently as a "House of Prayer and Learning" will be unlike any other religious venue in the world, its initiators say.
"It seemed to us that there was a very strong desire for the peaceful coming together of the religions," said Roland Stolte, one of two Protestant representatives on the board of the association behind the project.
Not by coincidence, it will stand at a location with a strong and long religious significance.
In 2007 archaeological excavations unearthed the foundations of four previous St Peter's churches that had stood on the site at different periods since the Middle Ages, Stolte told AFP in an interview.
The last one, which had a striking 100-metre-tall (328-foot) steeple and dated from the mid-19th century, was damaged in World War II and later demolished by the former East German communist state in the early 1960s.
A car park then occupied the site which the city authorities later handed back to the local Protestant community.
"We wanted to revive this place, not by building a church again but by constructing a place that says something about the life of religions today in Berlin," Stolte said.
Nearly 19 percent of Berlin's 3.4 million residents described themselves as Protestant, according to 2010 official data.
Some 8.1 percent said they were Muslim and 0.9 percent Jewish, while more than 60 percent said they did not adhere to any religion.
- 'Be patient' -
Pastor Gregor Hohberg said it had been crucial to also get the centre's Jewish and Muslim partners involved right from the start, well before work got underway on building it.
"From the beginning we wanted it to be an inter-religious project, not a place built by Christians in which Jews and Muslims would then be added," he said.
Imam Kadir Sanci, who's of Turkish origin, told AFP that a Catholic-Protestant church in western Germany had inspired him to dream that such a centre could be possible .
"When I was doing my Muslim theology studies in Frankfurt, I had seen in the neighbouring town of Darmstadt, a Catholic church and a Protestant church under the same roof," he said.
"I said to the priest it would be great to one day have a shared place with Muslims. But the priest told me 'be patient, it took us 600-plus years'," the imam said.
Architect Wilfried Kuehn, whose design for the new building was chosen in 2011 from around 200 entries in a competition, said it had posed many challenges that spanned architecture and theology.
"It was a challenge to try to combine the differences and the universal aspects. It was a question of not mixing the religions while ensuring mutual recognition," he said.
Each of the three religions will have its own equal-sized prayer space, all on the same floor, with each leading out to a common room where the congregations will be able to mix and chat.
Hohberg said that after much consideration, they had decided against a common prayer room "because that risked putting off more people than it would have attracted".
"And we want also to address the more conservative believers, to show that inter-religious dialogue is not only possible but important," he said.
However, financing must still be found, and organisers have sought to keep even that aspect of the ambitious project firmly at the grassroots level.
A crowdfunding campaign via the website www.house-of-one.org was launched Tuesday to raise the 43.5 million euros needed.
Donors are, among other things, invited to buy a "brick" for 10 euros.
"We wanted for this project to be base-driven," Stolte said, adding that they had sought out local partners such as Berlin's Jewish community or the Muslim group Forum for Inter-Cultural Dialogue rather than involving higher religious echelons.
For this reason, an upper limit has been set on individual donations -- at one percent of the total, or 435,000 euros.