Hagia Sophia, a UNESCO World Heritage Site, was the foremost cathedral in Christendom for 900 years before becoming one of Islam's greatest mosques for 500 years under the Ottoman Empire.
It was turned into a museum in 1935 by Mustafa Kemal Ataturk, founder of Modern Turkey.
Capital of Roman Empire
The Roman Emperor Constantine who embraced Christianity, made the ancient site between Europe and Asia the capital of the Roman Empire in 330 AD and it became known as Constantinople.
First destination of Ottoman Sultan Mehmet
When Ottoman Sultan Mehmet conquered Constantinople in 1453, his first destination was Hagia Sophia, the towering seat of Orthodox Christianity.
In front of what was then the largest church in the world, he knelt, sprinkled soil on his turban as a sign of humility and recited the Muslim prayer of faith, turning the church into a mosque.
'Should be renamed as mosque'
Turkey's President Tayyip Erdogan has previously said Istanbul's Hagia Sophia should be renamed as a mosque instead of a museum, triggering criticism from the European Union.
Conversion heavily disputed
Converting Istanbul’s crowning architectural treasure back into a mosque has been a cause close to the hearts of Turkish nationalists and Islamists for decades.
It has been opposed with equal fervour by Greece and Turkish liberals who argue the move would disrespect the history of the country’s Christian minority and erode the Republic’s secular character.
Converting it back into a mosque would sow division, the spiritual head of the world’s Orthodox Christians warned ahead of a Turkish court ruling on a building that has been a museum since 1934.
The court is set to rule on July 2 on a challenge to its current status that disputes the legality of its conversion into a museum in 1934 in the early years of the modern secular Turkish state founded by Mustafa Kemal Ataturk.
Warning to Turkey
The UN's cultural agency UNESCO warned Turkey against converting the Byzantine-era Hagia Sophia museum in Istanbul into a mosque, urging dialogue before any decision is taken.
UNESCO said that the Hagia Sophia was on its list of World Heritage Sites as a museum, and as such had certain commitments and legal obligations.
A top Turkish court on Friday revoked the Byzantine-era Hagia Sophia's status as a museum, clearing the way for it to be turned back into a mosque, according to reports.
The Council of State, which was debating a case brought by a Turkish NGO, cancelled a 1934 cabinet decision and ruled the UNESCO World Heritage site would be reopened to Muslim worshipping.