All these questions related to the transcendence of God, the role of the body in prayer, and divine light came to the fore in the middle of the fourteenth century. It is sometimes suggested that Barlaam was influenced by the nominalist philosophy popular in the West at that time, but it is more likely that he drew his teaching from Greek sources. Starting from the one-sided interpretation of Dionysius, he argued that God can be known only indirectly; Hesychasm, from his point of view, was wrong when he spoke of the direct experience of knowing God, for such an experience is inaccessible to man in earthly life. Referring to the bodily exercises practiced by the hesychasts, Barlaam accused them of a crudely materialistic understanding of prayer. He was also indignant at the claims of the hesychasts that they were able to behold the divine uncreated light: on this point he again accused them of crude materialism. How can man behold the divine essence with bodily eyes? The light that the hesychasts supposedly see is not the eternal light of divinity, but a temporary, created light.

St. Gregory Palamas (1296-1359), Archbishop of Thessalonica, rose to the defense of the hesychasts. He advocated a doctrine of the human person that justified the use of bodily exercises in prayer. He also argued, contrary to Barlaam, that the hesychasts did indeed see the divine uncreated light. To explain how this was possible, Gregory developed the doctrine of the distinction between essence and energies in God. It was Gregory who laid a solid dogmatic foundation for the practice of hesychasm, integrating it into Orthodox theology. Palamas' teaching was supported by the two councils held in Constantinople in 1341 and 1351, which, although they were local and not ecumenical councils, enjoyed an authority in Orthodox theology that was not much inferior to that of the Seven Ecumenical Councils. But Western Christendom has never officially recognized these two councils, although many Western Christians personally accept the theology of Palamas.

Gregory began by reaffirming the biblical teaching on the human person and the incarnation. Man is one and a single whole; not only the mind, but the whole man was created in the image of God. The body is not the enemy of the soul, but its assistant and co-worker. By taking on a human body in the incarnation, Christ "made the flesh an inexhaustible source of sanctification" [30]. Here Gregory picks up and develops ideas implicit in earlier texts, for example, in the homilia of Macarius; The same high understanding of the human body stands, as we have seen, behind the Orthodox doctrine of the veneration of icons. Gregory tried to apply this teaching about the human person to the hesychastic method of prayer: by emphasizing the role of the body in the process of prayer, the hesychasts (according to Gregory) were by no means guilty of crude materialism. They simply remained faithful to the biblical teaching about man as a whole. Christ took on human flesh and saved the whole man, therefore, the whole man must pray to Christ – body and soul together.

From this point, Gregory turns to the main problem: how to reconcile the two statements about the knowledge of God by man and the fact that God is by nature unknowable? Gregory's answer is: we know the energies of God, but not His essence. This distinction between essence (ousia) and energies in God goes back to the Cappadocian Fathers. "We know our God by His energies," writes St. Basil, "but we do not say that we can approach His essence. For though His energies descend upon us. His essence remains impregnable" [31]. Gregory accepts this distinction. With all the passion characteristic of every exponent of apophatic theology, he affirms the absolute unknowability of God in His essence. "God is not nature," writes Palamas, "for He is above all nature; is not that which is, for He is above all that exists... Nothing created by Him is in the slightest degree involved or will ever be involved in the higher nature or approached to it" [32]. But being extremely distant in His essence, God revealed Himself in His energies. The energies of God are not something that exists apart from Him, nor is it God's gift to people. No, it is God Himself in His action and in His manifestation to the world. God is wholly and completely present in each of the divine energies. As Gerard Manley Hopkins said, the world is marked by the majesty of God. The whole of creation is one gigantic burning bush, permeated but not destroyed by the ineffable and wondrous fire of divine energies [33].

It is through these energies that God enters into a direct and immediate relationship with humanity. In relation to us humans, the divine energy is in fact nothing but grace. Grace is not a "gift" of God, not an object given by God to mankind, but a direct manifestation of the Living God Himself, a personal encounter between creation and the Creator. "Grace . . . is the energy or the super-abundant procession of the one nature,.. in the aspect of communication to the created, their deification" [34]. When we say that the saints are transfigured or "deified" by the grace of God, we mean that they have been given a direct experience of seeing God himself. They know God, of course, in His energies, not in His essence.

God is Light, and therefore the experience of the divine energy takes the form of light. The light that the hesychasts contemplate is not some created radiance, but the light of the divinity itself, the light that surrounded Christ on Mount Tabor. This light, Palamas argues, is not sensual or material light, but it can be seen with natural sight (as Christ's disciples saw it during the Transfiguration). For when a person is deified, his bodily qualities, like his soul, are transformed. Therefore, the hesychastic vision of light is the true vision of God in His divine energies, and the hesychasts are quite right in identifying it with the uncreated light of Tabor.

In this way, Palamas preserves the transcendence of God, avoiding pantheism, into which careless mysticism easily slips, but at the same time he affirms the immanence of God, His constant presence in the world. God remains "wholly Other," but through His energies (which is God Himself) He enters into a direct relationship with the world. He is the Living God, the God of history, the God of the Bible, incarnate in Christ. When Barlaam excludes any possibility of direct knowledge of God and affirms the creation of the divine light of Tabor, he makes the gulf between God and humanity unbridgeable. Consequently, in his opposition to Barlaam, Gregory Palamas strives for the same thing that Athanasius and the Ecumenical Councils strove for: to preserve direct access to God, to defend the fullness of our redemption and deification. The doctrine that formed the essence of the controversy about the Trinity, the person of Christ, and the holy icons — this same doctrine constitutes the core of the hesychastic controversy.

"In the closed world of Byzantium," writes Dom Gregory Dicke, "after the sixth century not a single fresh movement of thought arose... Sleep began in the ninth century, and perhaps even earlier, in the sixth."[35] The Byzantine dogmatic disputes of the fourteenth century convincingly expose the falsity of such assertions. Of course, Gregory Palamas was not an innovative revolutionary, but firmly stood on the tradition of the past. Nevertheless, he was the primary creative theologian, and his writings testify to the fact that Orthodox theology did not lose its activity even after the eighth century and the Seventh Ecumenical Council.

Among the contemporaries of Gregory Palamas, Nicholas Cabasilas should be noted. Cabasilas is the author of the "Commentary on the Divine Liturgy", which has become a classic work on this topic in Orthodoxy, as well as a treatise on the sacraments "Life in Christ". Cabasilas's writings are marked by two characteristics: a lively sense of the person of Christ (who, in the words of Nicholas, is "nearer to us than our own soul"[36]) and an unfailing concentration on the sacraments. For Cabasilas, the mystical life is essentially life in Christ and in the sacraments. There may be a danger that mysticism will assume an individualistic and speculative character, but in Cabasilas it always remains Christocentric, sacramental, ecclesiastical. The writings of Cabasilas show how closely mysticism and sacramental life are connected in Byzantine theology. Palamas and his entourage never believed that mystical prayer made it possible to dispense with a normal, institutionalized church life.

The second council on the question of union was held in Florence in 1438-1439, which was personally attended by Emperor John VIII (reigned 1425-1448), together with the Patriarch of Constantinople and a large delegation of the Byzantine Church, as well as representatives of other Orthodox churches. The discussions were lengthy, and both sides made great efforts to reach a genuine agreement on the main issues. At the same time, it was difficult for the Greeks to discuss theological problems impartially: they knew that their political situation was desperate and that the only hope of defeating the Turks was help from the West. In the end, the formula of the union was worked out, which included the Filioque, theses on purgatory, "unleavened bread" and papal claims. It was signed by all the Orthodox present at the council, with the exception of one, Archbishop Mark of Ephesus, who was later canonized by the Orthodox Church. The Florentine Union was based on a dual principle: agreement in doctrinal matters and respect for the legitimate traditions and rites of each church. Thus, in matters of doctrine, the Orthodox recognized papal claims (although the formula of the union in its verbal expression was somewhat vague and ambiguous), recognized the thesis of the double procession of the Holy Spirit (although they were not required to introduce the Filioque into the text of the Divine Liturgy), and recognized the Roman Catholic teaching on purgatory (one of the points of divergence between East and West, which was revealed only in the thirteenth century). But as for the "unleavened bread," there was no need for unanimity: the Greeks were allowed to eat sour dough, while the Latins continued to eat unleavened dough.

But although the Florentine Union was celebrated throughout Western Europe—bells rang in all the parish churches of England—it proved to be no more viable than the Union of Lyons that preceded it. John VIII and his successor Constantine XI (the last Byzantine emperor and eighteenth in a row from Constantine the Great) remained faithful to the union; but they were powerless to impose it on their subjects, and did not even dare to proclaim it publicly in Constantinople until 1452. The decisions of the council were recognized only by an insignificant part of the Byzantine clergy and people. As if echoing what the emperor's sister said after Lyon, Grand Duke Luca Notaras declared: "I prefer to see a Muslim turban in the center of the city than a Latin mitre."

John and Constantine hoped that the Union of Florence would provide them with military support from the West, but the real help turned out to be negligible. On April 7, 1453, the Turks began an assault on Constantinople by land and sea. Outnumbered by more than 20 times, the Byzantines heroically held the line for seven long weeks. But the situation was hopeless. Early in the morning of May 29, the last Christian service was held in the Cathedral of St. Sophia. It was a joint service of the Orthodox and the Roman Catholics: at this decisive hour, the supporters and opponents of the Florentine Union forgot their differences. The emperor left the cathedral after communion and died fighting on the walls of the city. On the same day, in the late afternoon, the city was taken by the Turks, and the most famous temple in Christendom became a mosque.

It was the end of the Byzantine Empire. But not the end of the Patriarchate of Constantinople, much less the end of Orthodoxy.

CONVERSION OF THE SLAVS