"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 World Council of Churches' Interim Secretary General Ioan Sauca said.
The move drew worldwide criticism and concern, including from Greece, the United States and Russian Federation, as well as UNESCO and Pope Francis, who said he was hurt by the decision.
Hagia Sophia was a Byzantine church for nine centuries before the Ottomans converted it to a mosque.
The colossal Santa Sophia cathedral was turned into a mosque after the Ottomans conquered the city in 1453.More news: Coronavirus vaccine: Thailand plans trails for potential candidate in November
The World Council of Churches, which represents 350 Christian churches, said it had written to Erdogan expressing their "grief and dismay".
Losing the municipal elections a few months ago, prompted President Recep Tayyip Erdogan to convert Hagia Sophia in to a mosque, said Salha.
Mr Sauca said the museum status had been "a powerful expression" of Turkey's commitment to inclusion and secularism.
UNESCO said its World Heritage Committee would review Hagia Sophia's status, and Turkey's decision raised questions about the effect on its universal value as a site of importance transcending borders and generations.
"The opposite happens in Egypt, as everybody enjoys religious freedom and peaceful coexistence, our President El-Sisi grants titles to our Christian churches on a daily basis, and the political authorities as well as the state allow us freedom of worship, renovation, beautification and preservation of our churches", said Theodore.More news: One Plus Nord Price and Launch Date in India?
Erdogan has described foreign criticism over the proposal as an attack on Turkey's sovereignty.
Erdoğan's announcement came after a court cancelled a 1934 cabinet decision under modern Turkey's secularising founder Mustafa Kemal Atatürk to preserve the church-turned-mosque as a museum.
Built in the early 6th century, Hagia Sophia was the pinnacle of Byzantine architecture, and the largest church in the world for almost a thousand years.
Erdogan has assured that though Hagia Sophia will be open to prayers starting July 24, it will still be accessible to tourists across the world.More news: HK votes on 2nd day of weekend primary