[2] In his "Hagiography of Gregory of Sinai", he mentions two devotees, Jakov of Serres and Romylos of Vidin, then living and writing in Serbia.
According to Martin Jugie, contemporary historians depict Callistus I as a "doctrinaire and brutal man whose persecuting zeal it was necessary to restrain".
Callistus I died in August 1363 while he was en route to Serres as a member of the embassy of emperor John V Palaiologos seeking aid from Helena of Bulgaria, Empress of Serbia against the Ottoman Empire.
With another monk, Ignatius Xanthopoulos, with whom he had developed a life-long friendship at Mount Athos,[5][6][7][8] Callistus I composed the important Century, a tract of 100 sections on the ascetical practices of the Hesychastic monks; it was incorporated in the Philokalia of Nicodemus the Hagiorite and had a great influence on Orthodox spirituality.
In the Philokalia, the full title of the work is An exact rule and method with God's help for those who choose to live as hesychasts and monastics by the monks Kallistos and Ignatios Xanthopoulos, including testimonies from the saints.
The date of composition is suggested by the editor based on the references to Callistus I's clash with Nicephorus Gregoras at the final Palamite council.