Begun in 527 by Emperor Justinian, the Church of Saints Sergius and Bacchus in Istanbul was an early experiment in Byzantine architecture, with a large central dome supported by an octagonal base. The church is now a mosque called Küçük Ayasofya Camii (Little Hagia Sophia Mosque), named for its resemblance to the much larger Hagia Sophia built a few years later.
Saints Sergius and Bacchus are Christian Roman soldiers who were martyred in Syria in 303 AD. They became the patron saints of soldiers and their cult was very popular in Syria and beyond.
The Byzantine Emperor Justinian (r. 527-65) was among the saints' devotees. According to legend, when Justinian was a young man he was condemned to death for plotting against Emperor Anastasius. But Sergius and Bacchus appeared to the emperor in a dream, convincing him to release Justinian.
Justinian began construction on a church dedicated to Sergius and Bacchus immediately after becoming emperor himself in 527 AD; it was completed by 536 AD. The architect was Anthemius of Tralles, a mathematician and the author of a book on burning mirrors, the Paradoxographia. The chosen site was just inside the sea walls west of the Hormisdas Palace (where Justinian lived before ascending the throne), next to the Hippodrome.
The church was connected to a three-aisled basilica dedicated to St. Peter and St. Paul, which Justinian had begun to build in 519. None of it survives today. The Church of Sts. Sergius and Bacchus was built on an octagonal floor plan with a central dome, which inspired the design of the great Hagia Sophia, begun just a few years later in 532. The earlier church was therefore dubbed the "Little Ayasofya" (Küçük Ayasofya).
The church was converted into a mosque (camii) in the 16th century under Sultan Beyazit II and remains an active mosque today.
What to See
The former church is located on the south side of Istanbul next to the Hippodrome; a railway line (near Sirkeci Station) runs between the south wall of the church and sea wall.
The Küçük Ayasofya's interior is decorated and furnished as a mosque, with Arabic calligraphy and designs in blue painted on white walls. Originally, the walls and vault would have been completely covered in golden mosaics, like those that survive from this period in Ravenna, and probably frescoes as well.
The architecture of the building, however, survives fully intact from the Byzantine era. So too does the Greek dedicatory inscription around the central nave:
Other sovereigns have honoured dead men whose labour was unprofitable, but our sceptered Justinian, fostering piety, honours with a splendid abode the Servant of Christ, Begetter of all things, Sergius; whom not the burning breath of fire, nor the sword, nor any other constraint of torments disturbed; but who endured to be slain for the sake of Christ, the God, gaining by his blood heaven as his home. May he in all things guard the rule of the sleepless sovereign and increase the power of the God-crowned Theodora whose mind is adorned with piety, whose constant toil lies in unsparing efforts to nourish the destitute. (translation by Cyril Mango)
A fine view of the interior can be had from the gallery – stairs are to the right of the entrance.