John Calvin 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)

In the holy mount. He calls it the holy mount for the same reason the ground where God appeared to Moses was called holy. For wherever the Lord comes, as he is the fountain of all holiness, he makes all things holy by the fragrance of his presence. And by this way of speaking we are taught not only to receive God reverently wherever he shows himself, but also to prepare ourselves for holiness as soon as he comes near us, as the people were commanded when the law was proclaimed on Mount Sinai. And it is a general truth,

“Be ye holy, for I am holy, who dwell in the midst of you” (Leviticus 11:44; Leviticus 19:2).