Verse 34. - While he thus spake, there came a cloud, and overshadowed them: and they feared as they entered into the cloud. This luminous cloud, bright though it was, yet veiled the more intolerable brightness within. That such a bright cloud had the power of overshadowing and concealing, is not strange, for light in its utmost intensity hides as effectually as the darkness would do. God dwells in light inaccessible, whom therefore "no man hath seen, nor can see" (1 Timothy 6:16). Milton writes -

"Dark with excess of light." Philo speaks of the highest light as identical with darkness. Anselm thus understands the cloud here, quoting the words of 1 Timothy 6:16, referred to above, and then the words of Moses, "And Moses drew near unto the thick darkness where God was" (Exodus 20:21), and then this passage from the Transfiguration, and comments thus: "Illa caligo et ista nubes, atque ilia lux idem sunt" (see Archbishop Trench on "Transfiguration," in 'Studies in the Gospels,' 8). The fear which these eye-witnesses remember as one of their experiences that memorable night was a very natural feeling. As the cloud stole over the mountain ridge, and the glory-light gradually paled and waned, the sensation of intense pleasure and satisfaction, which we may assume to be the natural accompaniment of such a blessed scene, would give place to awe and amazement.

9:28-36 Christ's transfiguration was a specimen of that glory in which he will come to judge the world; and was an encouragement to his disciples to suffer for him. Prayer is a transfiguring, transforming duty, which makes the face to shine. Our Lord Jesus, even in his transfiguration, was willing to speak concerning his death and sufferings. In our greatest glories on earth, let us remember that in this world we have no continuing city. What need we have to pray to God for quickening grace, to make us lively! Yet that the disciples might be witnesses of this sign from heaven, after awhile they became awake, so that they were able to give a full account of what passed. But those know not what they say, that talk of making tabernacles on earth for glorified saints in heaven.While he thus spake there came a cloud,.... While Peter was making the above request, before an answer was returned, a cloud appeared, a very uncommon one, as a symbol of the divine presence: "and overshadowed them"; Jesus, Moses, Elias, and the disciples:

and they feared as they entered into the cloud; either as they themselves entered into it, that coming gradually over them, because of the glory of it, and the solemnity that attended it; or as Moses and Elias entered into it; and so the Syriac and Persic versions read, "they feared when they saw Moses and Elias enter into the cloud"; which took them out of their sight: just as the cloud received Jesus out of the sight of his disciples, when he ascended to heaven, Acts 1:9.