An excellent resource for this topic is The Uncreated Light by Solrunn Nes (Eerdmans 2007). Nes, a Norwegian Roman Catholic iconographer, examines icons of the Transfiguration from both artistic and theological perspectives. The oldest known representation of the event is found in the church of St Catherine's monastery at the foot of Mt Sinai. It was made under the sponsorship of the Byzantine emperor Justinian in the 6th century. On pp 69-71, Nes discusses the Transfiguration as eschatological sign.
When we turn our attention from the representation of Moses on the triumphal arch to the scene of the Transfiguration in the apse, we again are met by Moses. The same Moses, who, on Sinai, sought to see the transcendent God face to face, is shown on Tabor conversing face to face with he who "reflects the glory of God and bears the very stamp of his nature" (Heb 1:3). Moses has reached his goal. Together with Elijah he refers to the Old Covenant, confirms the New Covenant and points toward the contemplation of God in the age to come.
John Chrysostom (344-407) dwells on the idea of a gradual revelation of the mystery when he interprets the theophany at Sinai as a prefiguration of the theophany at Tabor which again is a prefiguration of the second coming of Christ. In a homily on the Transfiguration this eschatological aspect is described in the following way:
But if we will, we shall also behold Christ, not as they then on the mount, but in far greater brightness. For not thus shall he come hereafter. For whereas then, to spare his disciples, he disclosed only so much of his brightness as they were able to bear; hereafter he shall come in the glory of the Father, not with Moses and Elias only, but with the infinite hosts of the angels, with the archangels, with the cherubim, with those infinite tribes, not having a cloud over his head, but even heaven itself being folded up.
What the disciples glimpsed at Tabor will be thoroughly unfolded when the uncreated light breaks through into the created world. The uncreated light affects everything it touches and transforms the saints into the likeness of that which they see. The apostle John, who was eyewitness to the transfiguration, emphasizes this future perspective when he writes: "We know that, when he shall appear, we shall be like him; for we shall see him as he is" (1 John 3:2). Dionysius the Areopagite's vision of the glory of the world to come can be understood as a paraphrase in the following text:
when we have arisen incorruptible, immortal, and have attained the blessed Christ-like state, we shall be, as the Scripture says, "for ever with the Lord", filled, through the all-pure and holy contemplation, with the visible manifestation of God himself, shining through us with most radiant splendour, as it shone about the disciples in the Transfiguration.
Here we arrive at the fulfillment of the Christian promise of glory. The eschatological vision in the coming life is a completion of the mystical vision in this life.