Mark 9:1
And he said unto them, Verily I say unto you, That there be some of them that stand here, which shall not taste of death, till they have seen the kingdom of God come with power. And he said unto them, Verily I say unto you, That there be some of them that stand here, which shall not taste of death, till they have seen the kingdom of God come with power.
The promise that some standing with Jesus would not taste death until they had seen the kingdom of God come with power was fulfilled at the transfiguration as a miniature of Christ's kingdom. It was seen when Jesus appeared in heavenly glory and the Father's voice proclaimed Him to be the Son of God. The fashion of His countenance was changed and shone like the sun and His clothing became white and glistening, beheld by the disciples with fear and astonishment under an overshadowing cloud while God's voice spoke in terrible majesty identifying Jesus as His beloved Son to be heard. There they saw Christ clothed with the glory of Heaven in changed, radiant appearance, with the divine voice identifying Him.
Moses was present to represent those who will be raised from the dead at the second coming of Christ. Elijah was present to represent those who will be alive on earth at Christ's second coming and will be changed from mortal to immortal without dying. Elijah himself had been translated to Heaven without seeing death. Those represented by Elijah will be translated and taken to heaven without seeing death, changed to immortality at Christ's second appearing.