Theophylact of Ohrid — as excerpted in the Catena Aurea on Mark 8:34-38

“But because He had spoken of His glory, in order to shew that His promises were not vain, He subjoins, Verily I say unto you, That there be some of them that stand here who shall not taste of death, till they have seen the kingdom of God come with power. As if He said, Some, that is, Peter, James, and John, shall not taste of death, until I shew them, in my transfiguration, with what glory I am to come in my second advent; for the transfiguration was nothing else, but an announcement of the second coming of Christ, in which also Christ Himself and the Saints will shine.”