The Church of Theotokos Chalkoprateia (also known as St. Mary of Chalkoprateia or later as Acem Agha Mosque) was once one of the most important churches of Constantinople dedicated to the Virgin Mary. It was located about 100 meters west of Hagia Sophia, a short distance from the Basilica Cistern. It constructed entirely out of brick and was a three-aisled basilica with a polygonal apse, a narthex and an atrium. The church complex had other structures, including a surviving octagonal structure with fragmentary frescoes.

Starting in the Early Byzantine Era, there were three important churches dedicated to the Virgin Mary Theotokos located in Constantinople: the churches at Hodegetria, Blachernai, and Chalkoprateia. These churches were part of the growing emphasis on the cult of the Theotokos, which was connected to an effort to gather her relics in the capital.  Some sources suggest that all of these churches were built under the patronage of Pulcheria, sister of Theodosius II (408-450). We are told that, due to her efforts, the relics of the Theotokos - her shroud (robe), girdle (cincture) and an icon of her painted by St. Luke - came to reside permanently in Constantinople. However there is some confusion about her shroud, housed at Chalkoprateia and her maphorion (‘veil’) better attested at Blachernai. In establishing these three churches dedicated to the Theotokos, she set the location for her public veneration of the Theotokos and instituted weekly liturgies with processions, images, candles, and chanting of hymns. While Theotokos Chalkoprateia was not very big, especially when compared to its counterpart in Rome, Santa Maria Maggiore, it was nonetheless a very important Constantinopolitan church.

There are reasons to doubt Pulcheria’s connection to Theotokos Chalkoprateia, since Byzantine sources on this church are contradictory. We are also told both churches at Blachernai, and Chalkoprateia were built under the patronage of Verina, wife of Leo I (457-474). It has been suggested that Pulcheria’s church was destroyed in a fire in 476 and subsequently rebuilt or restored by Verina, although this has also been questioned. It was built in the district Chalkoprateia (“copper market”), whose name is said to derive from Jewish coppersmiths were lived there. The church was supposedly built near or over an older synagogue.

It was used as the patriarchal seat for five years (532-537) following the destruction of Hagia Sophia in the Nika Riots. During this period, the Council of 536, which condemned the Monophysites, was held here. Justin II (565-578) and his wife Sophia reportedly repaired the church, adorning it with gilded coffered ceiling and doors made of silver, electrum and gold. Three chapels were also added: the Hagia Soros, which housed the girdle of Virgin Mary, the chapel of St. Jacob Adelphotheos (or James, the Brother of the Lord) and a chapel dedicated to Christ, which housed the miraculous icon of Christ Antiphonetes. The Chapel of St. Jacob Adelphotheos housed the relics of the holy innocents, St. Zacharias (the father of John the Baptist), St. Simeon, and St. Jacob, the brother of the Lord.

Theotokos Chalkoprateia was repaired during the reign of Basil I (867-886), during which did a school (didaskaleion) was added to the church complex. It is also possible he added a dome above the church. Its relics appear in the West during the Latin occupation of the city when Franks controlled the church. The church probably was restored in the late 13th or early 14th century, during which time the remaining frescoes in the octagon date. It was converted into a mosque by Lala Hayreddin around 1484. Its waqf was established by Lala Hayreddin and expanded by Acemi Ahmed Agha.

It was damaged by a series of fires in the 18th and 19th centuries, but continued in use until the early 20th century. To date, only part of the church has survived. Private residences and other buildings were erected above the foundations of the church. A cruciform crypt and an octagonal structure adjoined to the north wall of the church’s atrium were discovered in the 1960s.