"And, behold, there talked with him two men, which were Moses and Elias: Who appeared in glory, and spake of his decease which he should accomplish at Jerusalem." (Luke 9:30-31)

This is a mysterious passage. Peter, James, and John watched in awe as Christ was "transfigured" before them as Christ had promised (Mark 9:2). But how could Moses and Elijah be there? Moses’ body had been buried by God in an unknown tomb in Moab some 1,500 years before, and no resurrection had yet taken place (Deuteronomy 34:5-6; 1 Corinthians 15:22-23). Elijah had been taken alive into heaven in a chariot of fire over 900 years previously (2 Kings 2:11).

The fact is that this whole experience was a remarkable vision! Jesus said, after it was over: "Tell the vision to no man, until the Son of man be risen again from the dead" (Matthew 17:9). Although they had just been awakened out of sleep (Luke 9:32), the disciples knew this was not a dream. All three had seen it together and "were sore afraid" (Mark 9:6).

This vision of the future kingdom was for the disciples’ encouragement (and for ours, as well), for the Lord had just been warning them of His coming death, as well as the cross which they, themselves, must take up to follow Him (Luke 9:23). The kingdom of God would come on earth, in all its future power and glory. But first, He must die and rise again, and they must be His witnesses of these things.

But when He did return in glory, there would be two groups of people sharing His glory with Him: Moses, representing the resurrected saints, and Elijah, the "raptured" saints. "The dead in Christ shall rise first: Then we which are alive and remain shall be caught up together with them in the clouds, to meet the Lord in the air: and so shall we ever be with the Lord" (1 Thessalonians 4:16-17). HMM http://www.icr.org/article/5916/