The World Council of Churches, which represents 350 Christian churches, said Saturday it wrote to Turkey's president expressing "grief and dismay" over his decision to turn the Hagia Sophia museum back into a mosque.
"Hagia Sophia has been a place of openness, encounter and inspiration for people from all nations and religions" since 1934 when it was turned from a mosque into a museum, the Geneva-based council's interim general secretary Ioan Sauca said in the letter.
The 1,500-year-old UNESCO-listed site was initially an Orthodox Christian cathedral that became a mosque following the Ottoman conquest of Istanbul in 1453.
On Friday, President Recep Tayyip Erdogan announced that the museum, one of the world's architectural wonders, would be reopened for Muslim worship as a mosque, sparking fury in the Christian community and neighboring Greece.
Erdogan's declaration came after a top Turkish court revoked a 1934 Turkish decision that turned the sixth-century Byzantine monument into a museum.
On Saturday, the council's statement underscored that "by deciding to convert the Hagia Sophia back to a mosque you have reversed that positive sign of Turkey's openness and changed it to a sign of exclusion and division."
The move would "inevitably create uncertainties, suspicions and mistrust, undermining all our efforts to bring people of different faiths together at the table of dialogue and cooperation," the statement said.
The council warned that the decision could also "encourage the ambitions of other groups elsewhere that seek to overturn the existing status quo and to promote renewed divisions between religious communities."