John Lightfoot on Matthew 17:2

[And was transfigured.] When Christ was baptized, being now ready to enter upon his evangelical priesthood, he is sealed by a heavenly voice for the High Priest, and is anointed with the Holy Spirit, as the high priests were wont to be with holy oil.

In this transfiguration, he is sealed for the high priest: for mark, 1. How two of the greatest prophets, Moses and Elias, resort to him. 2. How to those words, "This is my beloved Son, in whom I am well pleased," which also were heard from heaven at his baptism, is added that clause, "hear ye him": which compare with the words of Moses, concerning a prophet to be raised up by God, Deuteronomy 18:19, "Whosoever shall not hearken to my words, which I shall put into his mouth," &c. 3. How the heavenly voice went out of the cloud that overshadowed them, when at his baptism no such cloud appeared. Here that is worthy observing, which some Jews note, and reason dictates, namely, That the cloud of glory, the conductor of Israel, departed at the death of Moses; for while he lived, that cloud was the people's guide in the wilderness; but when he was dead, the ark of the covenant led them. Therefore, as that cloud departed at the death of Moses, that great prophet, so such a cloud was now present at the sealing of the greatest Prophet. 4. Christ here shines with such a brightness, nay, with a greater than Moses and Elias now glorified; and this both for the honour of his person and for the honour of his doctrine; both which surpassed by infinite degrees the persons and the doctrines of both of them. When you recollect the face of Christ transfigured, shining with so great lustre when he talked with Moses and Elias, acknowledge the brightness of the gospel above the cloudy obscurity of the law and of the prophets.

[For an interesting treatise on the subject of the transfiguration please see The Arthur C. Custance Library, The Doorway Papers "The Virgin Birth and the Incarnation, Part III: If Adam Had Not Died, Chapter 3: The Consequences of Immortality"]