I have blogged on the meaning of the “Transfiguration” in the context of Mark 9:2-7 and parallels in dialogue with Simon S. Lee and Candida Moss here. Why does 2 Peter 1:17 allude to this episode? For readers who accept the traditional authorship on the epistle, it may be that highlighting an event that was exclusively witnessed by the inner circle of Jesus’s three disciples (Peter, James, John) enhances Peter’s authority as an original eyewitness as opposed to other false teachers. On the other hand, readers that judge the epistle to be pseudonymous may reduce this reference to just an aspect of the “authorial fiction” presented in the letter.
However, there may be a deeper significance to this allusion. 2 Peter is not interested in the details about Jesus’s changed appearance or the presence of Moses and Elijah, but concentrates on the heavenly voice that identifies Jesus as the “Son.” That is, Jesus is the royal messianic heir of David who would rule over the nations in Psalm 2:7-9. Moreover, there is a literary seam that links the Transfiguration to an eschatological prediction of Jesus (Mark 9:1; Matthew 16:28; Luke 9:27), with Matthew referring to the coming (erchomai) of the Son of Man in his kingdom. Thus, like Matthew, 2 Peter sees the Transfiguration as a foretaste of Jesus’s return in glory and confirms that this doctrine is not a myth, in contrast to the scoffers who deny that the promised parousia (coming, advent) of Jesus will ever occur (2 Peter 3:2-3).