John Lightfoot on Matthew 17:5

[While he yet spake, behold, a cloud, &c.] Moses and Elias now turning their backs, and going out of the scene, Peter speaks his words; and as he speaks them when the prophets were now gone, "Behold, a cloud," &c. They had foretold Christ of his death (such is the cry of the Law and of the Prophets, that "Christ should suffer," Luke 24:44); he preaches his deity to his disciples, and the heavenly voice seals him for the true Messias. See 2 Peter 1:16,17.