The Hagia Sophia has stood as one of the greatest religious buildings in the world for nearly 1,500 years. During this time, it has taken many forms, first as a church and then a mosque, before finally becoming a museum in 1934 .But in July 2020, the Turkish president, Recep Tayyip Erdoğan, converted the Hagia Sophia back to a mosque following a ruling by the Turkish Council of State. This re-conversion was received with outrage across much of the world, but whether the decision was legal poses a much more difficult question.

This article analyzes Turkish domestic law and international law to conclude that there are grounds for questioning the legality of the Hagia Sophia’s re-conversion. It then addresses the need to better protect universal religious sites like the Hagia Sophia in the future. The Council of State relied on principles of the Islamic waqf endowment structure to declare the museum status of the Hagia Sophia illegal. But, in reality, waqf legal doctrine has been dynamic throughout history. Separately, the Turkish Constitution holds secularism as one of its greatest principles, which the re-conversion of the Hagia Sophia, cloaked with religious importance and symbolism, would seem to violate. On the other hand, international law on cultural heritage, freedom of religion, cultural rights, minority rights, and conquest provide much weaker constraints on the changed status of the Hagia Sophia. This is concerning given that Hagia Sophia is not the only universal religious site—a place of shared religious importance for two or more religions—in need of protection due to its unique history. In light of this, a future treaty on universal religious sites or the expansion of freedom of religion under the International Covenant on Civil and Political Rights could provide potential vehicles for such protection.