About the year 1337 Hesychasm attracted the attention of a learned member of the Orthodox Church, Barlaam of Calabria who at that time held the office of abbot in the Monastery of St Saviour's in Constantinople and who visited Mount Athos. Mount Athos was then at the height of its fame and influence under the reign of Andronicus III Palaeologus and under the 'first-ship' of the Protos Symeon. On Mount Athos, Barlaam encountered Hesychasts and heard descriptions of their practices, also reading the writings of the teacher in Hesychasm of St Gregory Palamas, himself an Athonite monk. Trained in Western Scholastic theology, Barlaam was scandalised by Hesychasm and began to combat it both orally and in his writings. As a private teacher of theology in the Western Scholastic mode, Barlaam propounded a more intellectual and propositional approach to the knowledge of God than the Hesychasts taught. Hesychasm is a form of constant purposeful prayer or experiential prayer, explicitly referred to as contemplation. Descriptions of the Hesychast practices can be found in the Philokalia, The Way of a Pilgrim, and St. John Climacus' The Ladder of Divine Ascent.
Barlaam took exception to, as heretical and blasphemous, the doctrine entertained by the Hesychasts as to the nature of the uncreated light, the experience of which was said to be the goal of Hesychast practice. It was maintained by the Hesychasts to be of divine origin and to be identical to that light which had been manifested to Jesus' disciples on Mount Tabor at the Transfiguration. This Barlaam held to be polytheistic, inasmuch as it postulated two eternal substances, a visible and an invisible God.
On the Hesychast side, the controversy was taken up by St Gregory Palamas, afterwards Archbishop of Thessalonica, who was asked by his fellow monks on Mt Athos to defend Hesychasm from the attacks of Barlaam. St Gregory himself, was well-educated in Greek philosophy. St Gregory defended Hesychasm in the 1340s at three different synods in Constantinople, and he also wrote a number of works in its defence.
In these works, St Gregory Palamas uses a distinction, already found in the 4th century in the works of the Cappadocian Fathers, between the energies or operations (Gr. energeies) of God and the essence (ousia) of God (see the Essence-Energies distinction). St Gregory taught that the energies or operations of God were uncreated. He taught that the essence of God can never be known by his creations even in the next life, but that his uncreated energies or operations can be known both in this life and in the next, and convey to the Hesychast in this life and to the righteous in the next life a true spiritual knowledge of God (see theoria). In Palamite theology, it is the uncreated energies of God that illumine the Hesychast who has been vouchsafed an experience of the Uncreated Light. Palamas referred to this experience as an apodictic (see Aristotle) validation of God rather than a scholastic contemplative or dialectical validation of God.
In 1341 the dispute came before a synod held at Constantinople and was presided over by the Emperor Andronicus; the synod, taking into account the regard in which the writings of the pseudo-Dionysius were held, condemned Barlaam, who recanted and returned to Calabria, afterwards becoming a bishop in the Roman Catholic Church.
One of Barlaam's friends, Gregory Akindynos, who originally was also a friend of St Gregory Palamas, took up the controversy, and three other synods on the subject were held, at the second of which the followers of Barlaam gained a brief victory. But in 1351 at a synod under the presidency of the Emperor John VI Cantacuzenus, Hesychast doctrine was established as the doctrine of the Orthodox Church.
Up to this day, the Roman Catholic Church has never fully accepted Hesychasm, especially the distinction between the energies or operations of God and the essence of God, and the notion that those energies or operations of God are uncreated. In Roman Catholic theology as it has developed since the Scholastic period circa 1100–1500, the essence of God can be known, but only in the next life; the grace of God is always created; and the essence of God is pure act, so that there can be no distinction between the energies or operations and the essence of God (see, e.g., the Summa Theologiae of St Thomas Aquinas). Some of these positions depend on Aristotelian metaphysics.
The contemporary historians Cantacuzenus and Nicephorus Gregoras deal very copiously with this subject, taking the Hesychast and Barlaamite sides respectively. Respected fathers of the church have held that these councils that agree that experiential prayer is Orthodox, refer to these as councils as Ecunemical Councils Eight and Nine. Father John S. Romanides, Hierotheos (Vlachos) of Nafpaktos, and the Very Rev. Prof. Dr. George Metallinos, Professor of theology at Athens Greece (see gnosiology).