Kallistos Ware (born Timothy Ware on 11 September 1934) is an English bishop within the Greek Orthodox Church under the Patriarch of Constantinople and one of the best-known contemporary Eastern Orthodox theologians. From 1982 he has held the Titular Bishopric of Diokleia. In 2007 the bishopric was made a titular metropolitan bishopric.

From 1966 to 2001, Ware was Spalding Lecturer of Eastern Orthodox Studies at the University of Oxford. He has authored numerous books and articles pertaining to the Orthodox Christian faith.

Born Timothy Ware in Bath, Somerset, England, he was educated at Westminster School in London (to which he had won a King's Scholarship) and Magdalen College, Oxford, where he took a double first in classics as well as reading theology. On 14 April 1958, at the age of 24, he embraced the Orthodox Christian faith (having been raised Anglican), travelling subsequently throughout Greece and spending a great deal of time at the Monastery of Saint John the Theologian in Patmos. He also frequented other major centres of Orthodoxy such as Jerusalem and Mount Athos. In 1966 he was ordained to the priesthood and was tonsured as a monk, receiving the name "Kállistos".

Gregory Palamas (Γρηγόριος Παλαμάς) (1296–1359) was a monk of Mount Athos in Greece and later the Archbishop of Thessaloniki known as a preeminent theologian of Hesychasm. The teachings embodied in his writings defending Hesychasm against the attack of Barlaam are sometimes referred to as Palamism, his followers as Palamites. Palamas is venerated as a Saint in the Eastern Orthodox Church. Though he is not widely venerated in the Roman Catholic Church, he is recognised as a saint and liturgically commemorated by the Eastern Catholic Churches of the Byzantine rite, which are in communion with Rome. Some of his writings are collected in the Philokalia. The second Sunday of the Great Lent is called the Sunday of Gregory Palamas in those Churches that commemorate him according to the Byzantine Rite. He also has a feast day on November 14.

Gregory was born in Constantinople in the year 1296. His father was a courtier of the Byzantine Emperor Andronikos II Palaiologos (1282–1328), but he died soon after Gregory was born. The Emperor himself took part in the raising and education of the fatherless boy. The Emperor had hoped that the gifted Gregory would devote himself to government service. St Gregory received his secular philosophical training from Theodore Metochites.