Today's feast of the Transfiguration, somewhat ignored by the Western churches, is of paramount importance in the Orthodox and Oriental churches.  This is because it is closely related to the doctrine of theosis, which lies at the heart of Eastern Christian spirituality.  On Mt Tabor, Jesus manifested the Uncreated Light, which Orthodox spiritual teachers maintain is God's divinity made visible to human eyes.  The Uncreated Light can also be manifested by Christians who have made substantial progress along the path to deification.  Probably the best-known example of this is St Seraphim of Sarov, the 19th-century starets whose illumination was described by his disciple Motovilov.

The significance of the Light during the actual Transfiguration is described in a sermon excerpted below by St Gregory Palamas (1296-1359).  Palamas, a monk of Mt Athos, defended the Athonite tradition of hesychasm against Barlaam of Calabria, a Greek from southern Italy whose theology was influenced by Roman Catholic scholasticism.  Barlaam held that the light experienced by Palamas and his brother monks was a created thing, if indeed it was not a product of self-delusion.  After a series of fiercely-contested theological debates, the Orthodox Church agreed with Palamas, and his opinions became part of official Orthodox teaching.  Palamas eventually became Archbishop of Thessalonica, where he delivered the following sermon. 

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Thus, the Light of the Transfiguration of the Lord  is not something that comes to be and then vanishes, nor is it subject to the sensory faculties, although it was contemplated by corporeal eyes for a short while upon an unconsequential mountaintop.  But the initiates of the mystery, the disciples of the Lord, at this time passed beyond mere flesh into spirit through a transformation of their senses, effectualized within them by the Spirit, and in such a way that they beheld what, and to what extent, the Divine Spirit had wrought blessedness in them to behold the Ineffable Light...

...That same Inscrutable Light shone and was mysteriously manifest to the apostles and the foremost of the Prophets at that moment, when the Lord was praying.  This shows that what brought forth this blessed sight was prayer, and that the radiance occurred and was manifest by uniting the mind with God, and that it is granted to all who, with constant exercise in efforts of virtue and prayer, strive with their mind towards God.  True beauty, essentially, can be contemplated only with a purified mind.  To gaze upon its luminance assumes a sort of participation in it, as though some bright ray etches itself upon the face...

...We believe that at the Transfiguration He manifested not some other sort of light, but only that which was concealed beneath His fleshly exterior.  This Light was the Light of the Divine Nature, and as such, it was Uncreated and Divine.  So also, in the teachings of the Fathers, Jesus Christ was transfigured on the Mount, not taking upon Himself something new nor being changed into something new, nor something which formerly He did not possess.  Rather, it was to show His disciples that which He already was, opening their eyes and bringing them from blindness to sight...

...Thus, this Light is not a light of the senses, and those contemplating it do not simply see with sensual eyes, but rather they are changed by the power of the Divine spirit.  They were transformed, and only in this way did they see the transformation taking place amidst the very assumption of our perishability, with the deification through union with the Word of God in place of this.