Theotokos Mosaic Hagia Sophia. Hagia Sophia was the largest and most admired cathedral in Christianity for almost a thousand years. The current building was originally constructed between 532 and 537 on the orders of the Byzantine Emperor Justinian. It became a mosque after the Ottoman conquest of 1453, and the four minarets were added in the subsequent years. It was secularized in 1931 and became a museum in 1935.