"Among those standing here there are some who will not taste death before they have seen the Kingdom of God"
Our Lord Jesus Christ took Peter, James and John onto the mountain to show them the glory of his divinity and teach them to know he was the Redeemer of Israel, as he had demonstrated through the prophets. He wanted to prevent them being scandalized, also, by the sight of his freely accepted sufferings, which he would suffer for us in his human nature. For they knew him as man but were ignorant that he was God; they knew him as Mary’s son, a man dwelling in the world with them, but on the mountain he showed them he was Son of God and God himself.
They had seen him eat and drink, work and rest, grow weary and sleep, undergo fear even to great drops of sweat: all things that seemed to bear no relation to his divine nature and seemed to belong only to his humanity. This was why he took them onto the mountain, so that the Father might call him his Son and show them that he was truly his Son and that he was God. He took them onto the mountain and showed them his Kingdom before making known his sufferings, his power before his death, his glory before his maltreatment, and his honor before his shame. Thus, when he was taken and crucified, his apostles would know that it had not been through weakness but by consent and of his own free will, for the salvation of the world.
St. Paul of the Cross