The Lord said to His disciples, “There are some standing here who will not taste death till they have seen the kingdom of God come with power” (Mark 9:1); and after six days He took Peter, James and John, and when they had ascended Mount Tabor He shone like the sun, and His clothes became white as light (cf. Matthew 17:1-2). When the disciples could look at it no longer or, rather, because they lacked the strength to gaze at the brightness, they fell prostrate to the earth (cf. Matthew 17:6). None the less, in accordance with the Savior’s promise they did see the kingdom of God, that divine and inexpressible light.
St Gregory of Nazianzos and St Basil call this light “divinity”, saying that “the light is the divinity manifested to the disciples on the Mount”, and that it is “the beauty of Him who is almighty, and His noetic and contemplatable divinity”.
St Basil the Great also says that this light is the beauty of God contemplated by the saints alone in the power of the divine Spirit; and again he writes, “On the mountain Peter and the sons of thunder saw His beauty shining more brightly than the sun; and they were privileged to receive with their eyes a foretaste of His advent.”
~St Gregory Palamas (Philokalia vol.4, pp.414-415)