The ruined church of Saint John (Agios Ioannis) is located in the village Episkopi, province Mylopotamos, and was built around the 13th century on the ruins of an early Christian basilica of the 5th century, as evidenced by architectural parts in second use. After the Arab occupation, it was the cathedral and the seat of the Diocese of Avlopotamos, giving the current name to the settlement of the Episkopi (Diocese). During the Venetian occupation it was given to the Latin church, while in the south a mansion was built for the archbishop. In 1299 the local Christian family of Alexios Kallergis agreed with the Venetians for the concession of the diocese, which was renovated and frescoed at his own expense.
From the temple today is preserved the north side and part of the southwest side. It was originally a three-aisled wooden-roofed basilica, which probably collapsed with the earthquake of 1303 and was rebuilt into a cruciform church with a dome supported by 4 columns. At one of the entrances there is the coat of arms of the Venetian bishop Sorreto dated from 1568, the year when the church was renovated.
Inside are preserved frescoes of Palaeologean style of the early 14th century, which have been associated by archaeologists with the chapel of the chrysoboulos of the monastery of Vrontochi in Mystras. Despite the deterioration from the weather conditions due to the lack of a roof, the frescoes of Panagia Platytera can still be seen among angels, the Preparation of the Throne, the Apostles of Pentecost and the Ascension.
On 3-12-1998, by decision of the Minister of Culture, the temple was declared a protected monument.