Situated next to the Harbour Corn Exchange, this is the first church to be dedicated to the Virgin Mary. It was also where the Third Ecumenical Council was held in 431 A.D., and so is considerably important to the development of Christian dogma.
The building in which it is housed, which is 260 m. in length and 30 m. in width, was used for scientific training, and for the theological training of the priests of Ephesus. The plan is that of a triple naved building, and in the 4th century the church was converted into a basilica with a central nave flanked by two aisles, when an apse was opened in the eastern I wall, and a Baptisterium added to the north side of an atrium to the west of the church. The central nave is the same width as the apse, while the flanking aisles are somewhat smaller. They are separated from the nave by two rows of columns, with geometric-designed balustrade panels between. There are mosaics on the floor of the narthex, to the western tip of the building, decorated with geometrical patterns, while the Atrium, which has one absidal wall is paved with stones of various kinds. The Baptisterium is circular in plan, and contains the baptismal pool in the centre. During the reign of the emperor Justinian (527 -565), further alterations led to the construction of a centrally-planned chapel surmounted by a single dome, between the apse and the narthex of the original church.
The cauldron in the centre was brought there from the Harbour Baths. In the 10th century a further church was added to the eastern front, with a small chapel being added to the southern tip of the church The council meeting held in this church in 431 agreed to accept as dogma the notion that Jesus, the son of the Virgin Mary was also the Son of God.