Moses and Elijah, the Law and the Prophets, came to the mountain to testify, and then the Father moved them aside. When the disciples looked up, they saw no one but Jesus only.
The Shadow
Six months before the cross, Jesus took Peter, James, and John up a mountain, and the glory He had laid aside broke through. Moses and Elijah appeared, talking with Him about His decease at Jerusalem, and the word beneath the English is exodus: the greater deliverance every shadow had been straining toward. Peter, overwhelmed, offered to build three tabernacles: one for Jesus, one for Moses, one for Elijah. Three equal honors: the Law, the Prophets, the Messiah.
Heaven interrupted him. A bright cloud, and a voice: “This is My beloved Son, in whom I am well pleased. Hear Him!” Not hear them. Hear Him. And when the disciples lifted their eyes, Moses was gone, Elijah was gone. They saw no one but Jesus only.
The Fulfillment
Moses and Elijah are not random visitors; they are the old covenant in person, the Law and the Prophets. Sinai's covenant arrived in a dark cloud around a mountain no one could touch; this cloud is bright. Moses's face once shone with a glory that faded, a fading radiance attached to a fading covenant; the glory on Jesus's face does not fade. John compressed it into one sentence: the law was given through Moses, but grace and truth came through Jesus Christ. A law can be delivered by a messenger. Grace had to arrive as a man. Paul adds the Law and the Prophets' true role: not the source of your righteousness, but its witnesses.
Him All Along
Every arrangement that offers you Jesus plus Moses, grace plus performance, is pitching Peter's three tents again, and the Father has already answered that suggestion. He overshadowed it with glory and took the other tents away. “This is My beloved Son, in whom I am well pleased” is now spoken over everyone hidden in Him. There is no one left on the mountain but Jesus. There never needed to be anyone else.