Istanbul's Hagia Sophia - a site at the heart of both the Christian Byzantine and Muslim Ottoman empires - is now at the centre of a court case over its future.
President Erdogan has proposed turning the museum, a UNESCO World Heritage Site, back into a mosque.
A Turkish court heard the case on Thursday (July 2) and will announce its verdict within 15 days.
The case disputes the legality of a decision by Kamal Ataturk, the father of secular modern Turkey, to convert the mosque into a museum in 1934.
Selami Karaman - a lawyer for the association that brought the case - said it use as a museum, quote, ''has hurt and saddened the Turkish people."
Turkish groups have long campaigned for Hagia Sophia's conversion into a mosque, saying this would better reflect Turkey's status as an overwhelmingly Muslim country.
Completed in the year 537, Hagia Sophia was the foremost church in Christendom for 900 years and then one of Islam's greatest mosques for 500 years after the Ottoman conquest of Istanbul in 1453.
The case has drawn an international backlash, with U.S. Secretary of State Mike Pompeo Wednesday (July 1) urging Turkey to keep it as a museum.
Greece said on Thursday Ankara risked opening up "a huge emotional chasm" with Christian countries if it pressed ahead with the mosque proposal.