Matthew 17:6

And were very much afraid. There were two causes that might produce this fear in the apostles, the cloud that overshadowed them, or the voice of God the Father, which they heard. Their human weakness could not bear such refulgent beams of glory, and trembling in every limb, they fall prostrate on the ground. (St. Jerome) --- The Almighty, it seems, was pleased to fulfil the wish of Peter, thereby to shew that Himself is the tent or pavilion, under the shade of which the blessed shall live for ever, and to sanction the public and explicit confession of Peter relative to the divinity of Jesus Christ, by his own no less public and explicit confession, joined with an express command to hear and obey him. St. John Chrysostom very justly remarks, that this voice was not heard till after the departure of Moses and Elias, that no possible doubt might exist to whom it was referred, and that it was to Christ only and to no other. --- Hear ye Him: i.e. as the law and the prophets are fulfilled and verified in Jesus Christ, your new legislator and prophet, you are to hear and obey Him in preference to either Moses or Elias, or any other teacher. (Haydock)