God's essence is unknowable to every creature. His energies are God Himself as known, encountered, communicated. To partake of God is to partake of the uncreated energies; the essence remains forever beyond participation. Theosis, the deification of the saints, is real because the energies through which God acts in the world are truly God, not a creature.

Behind this short sentence stand many centuries of patristic teaching. The grammar by which the Church speaks of God being one and yet truly participated, transcendent and yet truly immanent, was hammered out by the Cappadocians against Eunomius in the fourth century, refined by Cyril of Alexandria and Maximus the Confessor in the fifth and seventh, and given its mature dogmatic formulation by Gregory Palamas and the Constantinopolitan synods of 1341, 1347, and 1351 against Barlaam, Akindynos, and the later Latinizers.

Exodus 3:14: 'I AM WHO I AM.' The Name God gives to Moses at the burning bush, which St. Athanasius reads as the intimation of an incomprehensible essence: that God is, who is spoken of, while what He is as to essence remains unsaid.

Romans 1:20: 'The invisible things of Him, since the creation of the world, are clearly seen, being understood by the things that are made, even His eternal power and Godhead.' John 10:38: 'If ye believe not Me, believe the works.' The two passages give the same rule: God is known not by direct vision of the essence but through the works that are His.

Ephesians 1:19–20: 'the working of His mighty power, which He worked in Christ when He raised Him from the dead.' Philippians 3:21: 'according to the working by which He is able even to subdue all things to Himself.' 1 Corinthians 12:10–11: 'one and the same Spirit works all these things, distributing to each one individually as He wills.' St. Paul uses the Greek word *energeia* (energy, working, operation) as a real and concrete name for the activity of God in the Church and in creation.

St. Cyril of Jerusalem, in Catechesis VI on the Unity of God, asks whether a creature can comprehend the divine nature: 'If we cannot understand the throne that the prophet has described, how shall we be able to comprehend Him who sits upon it, the Invisible and Ineffable God? To scrutinise then the nature of God is impossible; but it is in our power to send up praises of His glory for His works that are seen.' (Catechetical Lectures VI.6, NPNF II.7.)

St. Athanasius, in his treatise on the decrees of Nicaea: 'Though to comprehend what the essence of God is be impossible, yet if we only understand that God is, and if Scripture indicates Him by means of these titles, we, with the intention of indicating Him and none else, call Him God and Father and Lord. When then He says, I am that I am, or when Scripture says God, we understand nothing else by it but the intimation of His incomprehensible essence Itself, and that He Is, who is spoken of.' (*De Decretis* 5.22, NPNF II.4.)

St. John Chrysostom, against the Eunomian claim that God's essence can be known by definition: 'For many of our conceptions about God, we are unable to express, as also many things we express, but have not strength to conceive of them.… We know that God is everywhere, but how, we no longer understand. Lo, we speak, and do not understand.' (Homily II on Hebrews, NPNF I.14.)

St. Basil the Great, in his Letter 189 to Eustathius: 'The divine nature is too exalted to be perceived as objects of enquiry are perceived… we are therefore of necessity guided in the investigation of the divine nature by its operations. Suppose we observe the operations of the Father, of the Son, of the Holy Ghost to be different from one another, we shall then conjecture, from the diversity of the operations, that the operating natures are also different.… Grant, then, that we perceive the operation of Father, Son, and Holy Ghost to be one and the same, in no respect showing difference or variation; from this identity of operation we necessarily infer the unity of the nature.' (Letter 189.6–7, NPNF II.8.)

St. John Chrysostom, on the prologue of John: 'For this man nowhere assigns the name of His essence (for it is not possible to say what God is, as to essence) but everywhere he declares Him to us by His workings. For this Word one may see shortly after called Light, and the Light in turn named Life.' (Homily II on John, NPNF I.14.)

St. Ambrose of Milan, on the same Pauline passage: 'You read that the invisible things of God, since the creation of the world, are understood through those things which have been made; His eternal power also and Godhead are estimated by His works. Wherefore also the Lord Himself says: If you believe not Me, believe at least the works. Believe, then, that the presence of the Godhead is there. Do you believe the working, and not believe the presence? Whence should the working proceed unless the presence went before?' (*On the Mysteries* III, NPNF II.10.)

St. Ephraim the Syrian, on the Transfiguration: 'On the mountain He showed His Apostles the glory of His divinity, concealed and hidden by His humanity. For they saw His face bright as lightning and His garments white as light. They saw two suns: one in the sky, as usual, and one unusually; one visible in the firmament and lightening the world, and one, His face, visible to them alone.'

'His garments white as light showed that the glory of His divinity flooded from His whole body, and His light shone from all His members. For His flesh did not shine with splendour from outside, like Moses, but the glory of His divinity flooded from Him. His light dawned and was drawn together in Him.… And He did not display the whole depth of His glory, but only as much as the limits of their eyes could encompass.' (Sermon on the Transfiguration of the Lord.)

The Tabor light is not a created splendour added to Christ's humanity from outside; it is the eternal glory of the Word, made visible to the disciples as their eyes are made able to bear it. This is the patristic ground on which Palamas later rests his case that the saints' vision of God is a vision of the uncreated.

St. Maximus the Confessor: 'And perhaps the holy Paul said he knew the knowledge of the Word of God in part, for he is only known to a limited degree on the basis of his activities. For knowledge concerning his substance and person in himself is to everyone alike, both to angels and humans.' (200 Chapters on Theology 2.76.)

St. John of Damascus, on the grammar of operation: 'In our Lord Jesus Christ there are two operations. For, in so far as He was God and consubstantial with the Father, He had the divine operation; in so far as He was made man and consubstantial with us, He had the operation of the human nature. However, one must know that operation is one thing, what is operative is another, which is operated another, and still another the operator. Operation, then, is the efficacious and substantial motion of the nature. And that which is operative is the nature from which the operation proceeds. That which is operated is the effect of the operation. And the operator is the one who performs the operation; the person, that is.' (*Exact Exposition of the Orthodox Faith* III.15, NPNF II.9.)

St. Cyril of Alexandria, in the *Dialogues on the Trinity* Book VII (PG 75, 1109 B–C), argues that the divine operation cannot be a creature. If anyone says that the Spirit, that is, the operation by which God works in creation, is itself a creature, then since God's operation is no other thing than He Himself, God Himself would be a creature, an abominable conclusion.

Cyril's principle: the divine being is simple and incomposite; it is not composed of nature and energy as though these were naturally other in the divine; the energy of God is God Himself proper to Him. To call the operation a creature is to call God a creature.

St. Gregory Palamas, on what 'an accident' can and cannot mean of God: 'An accident is that which comes into existence and passes out of existence.… But there is no such thing in God, because He remains entirely changeless. For this reason nothing can be attributed to Him that is an accident. Yet not all things said of God betoken His essence. For what belongs to the category of relation is also predicated of Him, and this is relative and refers to relationship with something else, and does not signify essence. Such is the divine energy in God. For it is not essence, nor an accident, even though it is called a kind of accident by some theologians, who mean to say simply this: that it is in God and that it is not essence.' (150 Chapters, Ch. 135.)

St. Gregory Palamas, on the unity of operation: 'There is one energy of the three divine hypostases not in the sense of similarity as in our case, but in being truly and numerically one.' (Kephalaia 138.)

St. Gregory Palamas, on the names: 'The substance of God is entirely unnameable since it is completely incomprehensible. Thus it is given names, improperly, on the basis of all its energies, although none of the names there differs from another in its denotation. For on the basis of each and all the names nothing other is named than the Hidden One, while what it is is in no way known. But in the case of the energies each of the names has a different meaning: for who does not know that creating, ruling, judging, guiding providentially, and God's adopting us as sons by His grace are different from one another?' (150 Chapters, Ch. 144.)

St. Gregory Palamas, summarising the patristic grammar in his Letter to Daniel of Ainos: 'There is a sense in which essence and energy are identical in God and a sense in which they differ from one another, as is clearly shown in the writings of the Fathers.' (Letter to Daniel of Ainos 3–4.) And in the Letter to Gabras: the essence and energies are distinguished by 'dividing the indivisible conceptually, and recognising their difference for us, but not distancing them and separating them from each other.' (Letter to Gabras, sec. 29–30.)