Charles Ellicott Commentary 2 Peter 1:18

"and this voice we [ourselves] heard borne out of heaven, when we were with him in the holy mount." — 2 Peter 1:18 (ASV)

And this voice which came from heaven we heard.—Rather, And this voice we heard carried from heaven: We were ear-witnesses of the voice coming from heaven, as we were eye-witnesses of His majesty. It was no vision, it was no hallucination. We all heard, and we all saw; so that I have the highest authority for what I would now impress upon you. A voice which I myself heard carried from heaven to earth, in the midst of glory which I myself saw, foretelling the glory that is yet to come.

In the holy mount.—It is, perhaps, not even “partly right” to say that the epithet “holy” indicates a view of the event later than that of the Evangelists, and points to a miracle-loving age. Rather, it indicates a view many centuries older than the Evangelists—that wherever God had specially manifested Himself was “holy ground” (Exodus 3:5; Joshua 5:15. Compare to Genesis 28:16-17; Exodus 19:12; Acts 7:33). The expression would be natural to any Jew speaking of the Transfiguration. (See Introduction, in the place cited).