Jesus took the three apostles up to the mountain for three reasons: first, to show them the glory of his divinity, then to declare himself Israel’s redeemer as he had already foretold by the prophets, and thirdly to prevent the apostles’ being scandalized at seeing him soon afterward enduring those human sufferings which he had freely accepted for our sake.
The apostles knew that Jesus was a man; they did not know that he was God. To their knowledge he was the son of Mary, a man who shared their daily life in this world. On the mountain he revealed to them that he was the Son of God, that he was in fact God himself. Peter, James, and John were familiar with the sight of their master eating and drinking, working and taking rest, growing tired and falling asleep, experiencing fear and breaking out in sweat. All these things were natural to his humanity, not to his divinity.
He therefore took them up onto the mountain so that they could hear his Father’s voice calling him Son, and he could show them that he was truly the Son of God and was himself divine.
He took them up onto the mountain in order to show them his kingship before they witnessed his passion, to let them see his mighty power before they watched his death, to reveal his glory to them before they beheld his humiliation. Then when the Jews took him captive and condemned him to the cross, the apostles would understand that it was not for any lack of power on his part that Jesus allowed himself to be crucified by his enemies, but because he had freely chosen to suffer in that way for the world’s salvation.
He took them up onto the mountain before his resurrection and showed them the glory of his divinity, so that when he rose from the dead in that same divine glory they would realize that this was not something given him as a reward for his labor, as if he were previously without it. That glory had been his with the Father from all eternity, as is clear from his words on approaching his freely chosen passion: “Father, glorify me now with the glory I had with you before the world was made.”
St Ephrem the Syrian