The Transfiguration of Christ appeared to some of the disciples to show them that He is neither Elijah nor John the Baptist as people say, but He is Christ the Son of the living God.
To show them the glory of his Divinity before his suffering, so they would not be suspicious of Him at his crucifixion.
The disciples who were with Him on the mountain, representing the Church of the Battlers’. Moses and Elijah the prophet representing the Church of the Victorious. This proofs our Church's connection with angels and the Heavenly Fathers.
Mark 9: 2-13
2 Now after six days Jesus took Peter, James, and John, and led them up on a high mountain apart by themselves; and He was transfigured before them.
3 His clothes became shining, exceedingly white, like snow, such as no launderer on earth can whiten them.
4 And Elijah appeared to them with Moses, and they were talking with Jesus.
5 Then Peter answered and said to Jesus, “Rabbi, it is good for us to be here; and let us make three tabernacles: one for You, one for Moses, and one for Elijah”—
6 because he did not know what to say, for they were greatly afraid.
7 And a cloud came and overshadowed them; and a voice came out of the cloud, saying, “This is My beloved Son. Hear Him!”
8 Suddenly, when they had looked around, they saw no one anymore, but only Jesus with themselves.
9 Now as they came down from the mountain, He commanded them that they should tell no one the things they had seen, till the Son of Man had risen from the dead.
10 So they kept this word to themselves, questioning what the rising from the dead meant.
11 And they asked Him, saying, “Why do the scribes say that Elijah must come first?”
12 Then He answered and told them, “Indeed, Elijah is coming first and restores all things. And how is it written concerning the Son of Man, that He must suffer many things and be treated with contempt?
13 But I say to you that Elijah has also come, and they did to him whatever they wished, as it is written of him.”