Peter answereth, and saith to Jesus. We learn from St. Luke 9:33 that this happened just as Moses and Elijah were departing. Peter was excited, and there was fear mingled with his excitement. He was bewildered.
His first idea was to seek that they might remain, for he saw that they were just preparing to depart. Theophylact says upon this, "Do not say with Peter, 'It is good for us to be here;' for it behoves us ever, whilst in the flesh, to be advancing, and not to remain in one stage of virtue and contemplation, but to pass on to other degrees" It is, perhaps, too curious a question to ask how the three disciples knew them to be Moses and Elijah.
The same Divine power which presented them with a vision of the other world gave them an intuitive knowledge on the subject. And we may, perhaps, infer from hence that in that world to come there will be not only recognition, but knowledge, at once imparted, of those whose faces we have not seen "in the flesh."
St. Luke 9:32 says that Peter and his companions "were heavy with sleep ( βεβαρημένοι ὕπνῳ)." It is probable that the Transfiguration took place at night. The whole manifestation would be rendered more conspicuous and striking amidst the darkness and stillness of night.
But St. Luke is careful to add, "when they were fully awake ( διαγρηγορήσαντες)." This word might be rendered, "having remained awake." But whichever translation be adopted, the intention of the evangelist is evidently to show that it was not in a dream or a vision of the night that they saw this.
It was a great reality, on which they looked with open eyes.