The author uses the reflexive pronoun "themselves" here to emphasize that they were alone and that only Jesus, Peter, James, and John went up the mountain. (See: [[rc://en/ta/man/translate/figs-rpronouns]])
# he was transfigured before them
When they looked at him, his appearance was different from what it had been.
This can be stated in active form. Alternate translation: "his appearance had changed" or "he appeared very different" (See: [[rc://en/ta/man/translate/figs-activepassive]])