It appears that our Lord separated himself from all the disciples except Peter and James and John, and then the bitter agony began. He began to be greatly amazed, and sore troubled ( ἐκθαμβεῖσθαι καὶ ἀδημονεῖν).
These two Greek verbs are as adequately expressed above as seems possible. The first implies "utter, extreme amazement;" if the second has for its root ἄδημος, "not at home," it implies the anguish of the soul struggling to free itself from the body under the pressure of intense mental distress.
The three chosen disciples were allowed to be witnesses of this awful anguish. They had been fortified to endure the sight by the glories of the transfiguration. It would have been too much for the faith of the rest.
But these three witnessed it, that they might learn themselves, and be able to teach others, that the way to glory is by suffering.