Matthew Poole and continuators on 2 Peter 1:18

18 And this voice which came from heaven we heard r, when we were with him in the holy mount.

I, and Iames, and Iohn. The Apostle avoucheth himself to have been an Ear-witness, as well as; Eye-witness of Christ's Glory, hereby intimating that there was as much certainty of the Gospel, even in an human way, as could possibly be obtained of any thing that is done in the World, seeing Men can be humanly certain of nothing more, than of what they perceive by their Senses: compare 1 Iohn 1.1, 3.

So called, not because of any inherent Holiness in it, but because of the extraordinary Manifestation of God's Presence there, in the same sense as the Ground is called holy where God appeared to Moses and Ioshua, Exod. 3.5. Iosh. 5.15.