More on the Trinitarian nature of the feast of the Transfiguration - selections from Vespers and Matins:
Christ, the Light that shone before the sun, who in the body went about the earth, having fulfilled before His Crucifixion, as befitted His divine majesty, all things pertaining to His fearful dispensation, this day has mystically manifest the image of the Trinity upon Mount Tabor. For taking the three disciples He had expressly chosen, Peter, James, and John, He led them up into the mountain; and for a short time He concealed the flesh He had assumed, and was transfigured before them, making manifest the excellence of the original beauty, though not in its full perfection. For while giving them full assurance He also spared them, lest at the sight they should lose their lives: yet they saw as much as their bodily eyes were able to receive. He likewise called before Him the chief prophets Moses and Elijah, who testified to His divinity, that He is indeed the true brightness of the essence of the Father, the Ruler of the living and the dead. Therefore a cloud wrapped them like a tent; and out of the cloud from above loudly sounded the voice of the Father, testifying and saying: This is my beloved Son, whom I have begotten without change from the womb before the morning star: Him have I sent to save those who are baptized in the name of Father, Son, and Holy Spirit, and who confess with faith that the One Power of the Godhead is indivisible. Listen to Him. And, O Christ our God, supreme in goodness, who loves mankind, shine upon us with the light of Your unapproachable glory and make us worthy to inherit Your never-ending Kingdom.
You ascended the mount with Your Disciples, and shone in the glory of the Father. Moses and Elijah were present with You; for the law and the prophets serve You, as You are God. And the Father designated for You natural prophetship, calling You “Son.” Therefore, we praise Him, with You, and with the Holy Spirit.
The pillar of fire plainly showed to Moses Christ transfigured, and the cloud pointed clearly to the grace of the Spirit that overshadowed Mount Tabor.
Come and hear me, O you peoples: going up into the holy and heavenly mountain, let us stand in spirit in the city of the living God, and let us gaze with our minds at the spiritual Godhead of the Father and the Spirit, shining forth in the Only-begotten Son.
Today on Tabor in the manifestation of Your light, O Word, unaltered Light from the Light of the unbegotten Father, we have seen the Father as Light and the Spirit as Light, guiding with light the whole creation. (Exaposteilarion of the feast)