Saint Callistus I, Patriarch of Constantinople, at first struggled on Athos under the spiritual guidance of St Gregory of Sinai (August 8), whose Life he wrote. In 1350, he was elected as Patriarch of Constantinople, serving in that position during the reign of the emperors John Kantakuzenos (1341-1355) and John Paleologos (1341-1376).
In 1354, he withdrew to live in silence at the monastery he had built in honor of St Mamas at Tenedos. Later, he was elevated to the Patriarchal throne once again (1355-1363). The holy Patriarch Callistus reposed in the year 1363 in Serbia, where he had travelled with an embassy of Emperor John Paleologos. St Callistus is also known as a spiritual writer, and his edifying works appear in the PHILOKALIA with the writings of his close friend Ignatius of Xanthopoulos.