Emperor John VI Kantakouzenos, a staunch follower of Palamism, the Hesychast doctrine of Gregory Palamas, had befriended Demetrios Kydones as a young man and had employed him as his Imperial Premier or Mesazon (1347–1354) at the age of 23; at the Emperor's request, Kydones began to translate Western polemical works against Islam, such as the writings of the Dominican Ricoldo da Monte Croce, from Latin into Greek, and which the Emperor John VI Kantakouzenos used as references in his own writings against Muhammad and Islam (although his own daughter was married to the Turkish Muslim Emir Orhan of Bithynia).
Prochoros admired and translated some of the works of Augustine of Hippo and Thomas Aquinas, but parted company with Kantakouzenos by becoming an argumentative anti-Palamite.
On retiring from public office in 1354, Demetrios Kydones went to Italy where he studied the writings of the leading medieval philosophical theologians, and made Greek translations of the major works of Western writers, including tracts by Augustine of Hippo (5th century) and Thomas Aquinas' Summa theologica.
[3] In 1369, Emperor John V Palaiologos recalled Kydones to Constantinople and named him Imperial Prime Minister or Mesazon, the second time he held this position, 1369–1383.
With the support of his younger brother Prochoros, Demetrios opposed as polytheistic or pantheistic the Palamites' commitment to hesychasm (Greek, silence or stillness), at the time a controversial practice of mystical contemplation through uninterrupted prayer, taught by the Orthodox monks of Mount Athos and articulated by the 14th-century ascetic theologian Gregory Palamas.
He is the author of the moral philosophical essay De contemnenda morte ("On Despising Death"), an Apologia for his conversion to Catholicism, and a voluminous collection of 447 letters, valuable for the history of Byzantine relations with the West.
One of the principal documentary sources for the Eastern Roman Empire's gradual submission to the Turks is Kydones' Symbouleutikoi ("Exhortations"), urging the Byzantine people to unite with the Latins in order to resist the Turkish onslaught.