Adam Clarke on 2 Peter 1:18

And this voice - we heard - That is, himself, James, and John heard it, and saw this glory; for these only were the εποπται, beholders, on the holy mount. It is worthy of remark that our blessed Lord, who came to give a new law to mankind, appeared on this holy mount with splendor and great glory, as God did when he came on the holy mount, Sinai, to give the old law to Moses. And when the voice came from the excellent glory, This is my Son, the beloved One, in whom I have delighted; hear him: the authority of the old law was taken away. Neither Moses nor Elijah, the law nor the prophets, must tabernacle among men, as teaching the whole way of salvation, and affording the means of eternal life; these things they had pointed out, but these things they did not contain; yet the fulfillment of their types and predictions rendered their declarations more firm and incontestable. See below.