He said to them, “Most certainly I tell you, there are some standing here who will in no way taste death until they see God’s Kingdom come with power.”
After six days Jesus took with him Peter, James, and John, and brought them up onto a high mountain privately by themselves, and he was changed into another form in front of them. His clothing became glistening, exceedingly white, like snow, such as no launderer on earth can whiten them. Elijah and Moses appeared to them, and they were talking with Jesus.
Peter answered Jesus, “Rabbi, it is good for us to be here. Let’s make three tents: one for you, one for Moses, and one for Elijah.” For he didn’t know what to say, for they were very afraid.
A cloud came, overshadowing them, and a voice came out of the cloud, “This is my beloved Son. Listen to him.”
Suddenly looking around, they saw no one with them any more, except Jesus only.
As they were coming down from the mountain, he commanded them that they should tell no one what things they had seen, until after the Son of Man had risen from the dead. They kept this saying to themselves, questioning what the “rising from the dead” meant.
They asked him, saying, “Why do the scribes say that Elijah must come first?”
He said to them, “Elijah indeed comes first, and restores all things. How is it written about the Son of Man, that he should suffer many things and be despised? But I tell you that Elijah has come, and they have also done to him whatever they wanted to, even as it is written about him.”
The transfiguration story is strategically positioned in Mark’s Gospel. The first phase of Jesus’ ministry commenced with the heavenly acclamation, “You are my Son, the beloved one. I am very pleased with you” (1:11). At every point of his subsequent ministry this Sonship is affirmed, implicitly (“Who indeed is this that even the wind and the sea obey him?”) or explicitly(“I know who you are, the Holy One of God!). But with the announcement of the passion, a new phase in Jesus’ ministry is introduced. The Marcan Evangelist must assure his readers that Jesus still enjoys heaven’s favor and that his mission still has validity. The Evangelist achieves this by a heavenly endorsement where not only do we hear God again claiming Jesus as his Son, but we witness Jesus in the company of two of ancient Israel’s greatest figures: Moses and Elijah.
(Craig E. Evans, The Lectionary Commentary: The Third Readings: The Gospels, p. 240)