The Basilica of St. John was one of the largest sacral buildings of the Byzantine Empire. It was an early Christian basilica dedicated to the apostle John and donated by Emperor Justinian. Its remains are located on the slope of the Ayasoluk Hill, near the centre of Selçuk, just below the Byzantine-Seljuk fortress.
Since the 7th century, the centre of Byzantine Ephesus had shifted to the hill about 3.5 km away from ancient Ephesus.

When the Arabs attacked Ephesus in the 7th/8th century, a defensive wall was built around the church. The wall had 20 towers and three gates. Sarcophagi were built into the wall above the archway.
One of these sarcophagi, now in England, depicted Achill's persecution of the Hector. That is why this gate was called the "Gate of Persecution".

Since the middle of the second century, there has been an early Christian tradition that allows the apostle John, equal to Jesus' favourite disciple, to move with Mary to Ephesus, where he is said to have written the fourth Gospel. According to scientific findings, however, the Gospel of John came into being much later.
The followers of Christianity were expelled from Jerusalem in the years 37 to 42. After the beheading of St. Jacob, brother of John, John saw a great danger and left Jerusalem with Mary. In 41 they migrated through Syria to Anatolia as far as Ephesus. Ephesus was then an important centre of missionary activity. After Paul was executed in 62 A.D., he took over his office according to early Christian tradition.

According to the old church tradition, St. John was arrested by Emperor Domitian (81-96) during the persecution of Christians.
A legend from the apostle's life is particularly well known: Since the priests of the temple of Artemis in Ephesus feared that John would lose too many followers, the high priest gave him the choice of either sacrificing in the temple or emptying a cup of poison from which two criminals had previously died.
When John struck the sign of the cross over the cup, the poison escaped in the form of a snake. Then John spread his mantle over the two dead and brought them back to life. The high priest himself is said to have converted to Christianity.
The news of this miracle spread rapidly and Domitian was so afraid that he released John and exiled him to the Greek island of Patmos. After Domitian's death John is said to have returned to Ephesus with Mary and to have written the Gospel of John.

John supposedly died in the first years of the reign of Emperor Trajan (101 AD). He is the only apostle who died a natural death. He was buried, as he wished, in a cemetery above the city. A mausoleum in the form of a cross vault supported by four columns was erected above the site of the tomb.

After Christianity became a state religion in the later 4th century, a church was built over John's tomb. The stones and marble of the destroyed temple of Artemis were used for the construction.
Emperor Justinian (527-565) replaced this church with a three-nave basilica in the form of a Latin cross. The basilica is 130 m long and 40 m wide and was one of the seven large churches of Asia Minor. Together with the Hagia Sophia, it was one of the largest late antique or early Byzantine churches and was a place of pilgrimage in the Middle Ages.

After the conquest by the Seljuks in 1330, St. John's Church was temporarily used as a mosque.
After the Isabey Mosque was built in 1375, the basilica lost its importance as a mosque. In the 14th century an earthquake damaged the building, in 1402 Timur's cavalry completely destroyed the church.

The baptismal font in the Baptistery

The baptismal act was performed in the Baptistery: it is an octagonal room dedicated to John the Baptist. In a round baptismal font embedded in the ground, the baptized person climbed three steps coming from the west to receive the baptism. Afterwards he left the basin, again via three steps to the east.