John Chrysostom — as excerpted in the Catena Aurea on Mark 9:1-8

“f Again, Peter neither comprehended that the Lord worked His transfiguration for the shewing forth of His true glory, nor that He did this in order to teach men, nor that it was impossible for them to leave the multitude and dwell in the mountain. It goes on, For they were sore afraid. But this fear of theirs was one by which they were raised from their usual state of mind to one higher, and they recognised that those who appeared to them were Moses and Elias. The soul also was drawn on to a state of heavenly feeling, as though carried away from human sense by the heavenly vision.”