Mark and Luke relate the same without any considerable variation, only Luke saith, they feared as they entered into the cloud. It seemeth that the cloud did encompass them, so as they seemed all as if they had been within the cloud. This still increased their fear. It is observable, that God did very often make his appearances to people in a cloud, making the clouds his chariots, Exo_16:10 40:34 Num_11:25Psa_104:3 to teach us humility, not to pry too much into his secrets, who covereth himself with thick darkness, and likewise to consult our weakness, who are not able to behold him as he is. This is said to be a bright cloud, so differing from the cloud in which he appeared under the law, but without doubt it had something of a shadow in it, and was chosen of God for some abatement of the brightness of his glory. This cloud encompasses Christ, Moses, and Elias, and also Peter, James, and John.
And behold a voice out of the cloud: they saw no visible shape, no more did the Jews, Deu_4:15, only, as St. Peter (who saw it) expresses it, 2Pe_1:17, there came such a voice from the excellent glory. He speaks of this very time, as may appear from 1Pe_1:18. The voice is the same which was heard upon the baptism of Christ, Mat_3:17; only there is added to it, hear ye him: you need no Elias to instruct you, hear him. Thus Moses saw what he had before prophesied of, Deu_18:15,18, fulfilled: he in this ministry as a servant in the house of God had prophesied, that the Lord would raise up a prophet from amongst their brethren like unto him, and put his words into his mouth and he should speak unto them all that God should command him, Deu_18:18; and, Deu_18:15, unto him ye shall hearken. God had now fulfilled that word, and he declares that this prophet was his Son, his beloved Son, and commands them to hear him. Which words establish Christ as the only Doctor and Teacher of his church, the only one whom he had entrusted to deliver his truths and will to his people, the only one to whom Christians are to hearken: nor doth this destroy the ministers of the word, who are no more than the interpreters of what he hath said, and are no more to be regarded than as by them we hear Christ speaking more plainly and frequently unto us. This appearance of God from time to time in a cloud, and that not in any visible shape, but in an excellent glory, causing a voice to be heard, lets us see the audaciousness of those who by any pictures or images pretend to make any representation of any person in the Trinity. And this command from God to us to hear Christ, lets us also see the audacity of those who take upon them to impose upon Christians what Christ never spake.