The apostles" message was essentially that Jesus was the Christ (i.e, God"s promised Messiah; cf. 1 John 5:1). God had revealed this clearly at Jesus" transfiguration when He had announced that Jesus was His beloved Son ( Matthew 17:5; Mark 9:7; Luke 9:35). Peter referred to that event to establish the credibility of his witness and that of the other apostles. The terms "honor," "glory, "Majestic Glory," and "holy mountain" all enhance the special event that was the Transfiguration. [source][source][source]
"The author is ... pointing out to his readers that the Transfiguration, to which the apostles bore witness, is a basis for the expectation of the Parousia.... [source][source][source]
"The emphasis of the account is that God himself has elected Jesus to be his vicegerent, appointed him to the office and invested him with glory for the task." [1][source]