The transfiguration is a very special event in the life of Jesus and the disciples who were following him. The event itself had several significances that are important, especially for the disciples to understand. However, something struck me about this text that is not really the main point. The transfiguration gives us a small peek, a special glimpse if you will, into the afterlife - particularly of Moses and Elijah.
While the appearing of Moses and Elijah are for a particular purpose and by design of God, note one small fact that may not have hit you right away: Moses and Elijah in their full personalities and consciousness are actually here appearing in history per God's redemptive plan. Far from being simple "tools" accomplishing God's purposes, Moses and Elijah get the opportunity to speak with the Lord Jesus Christ face to face.
What's more, look at their manner of speech. The same account in Luke 9 tells us that they were speaking about Jesus going to the cross. However, these men are actually simply standing and speaking to the king of the universe. One gets the impressions that this is a somewhat regular occurrence for saints who have gone to glory. Moses and Elijah aren't falling on their faces or weeping before the Lord, they are chatting with him about the great plan of redemption.
I don't mean to diminish the content of which they were speaking, nor the importance of their appearance and help, rather I simply strive to get at the fact that speaking with our God and maker and our Lord Jesus Christ is something that Moses and Elijah seem fairly accustomed to; and is likely something we will do regularly in the afterlife. Sometimes I get the impression in my head that heaven is a vast gathering of people, and God is still somewhere far off beyond the throngs of saints. Maybe I will get to see him one day in eternity, but I'll probably have to wait in a very long line. I would submit that this isn't a correct perspective. I don't know exactly what heaven will be like, but I do know that I will be closer to my God and the saints than I have ever been on this earth. I think J. C. Ryle gives a great closing thought for us to ponder on this text:
"Now we have in the transfiguration the clearest evidence that the dead will rise again. We find two men appearing on earth, in their bodies, who had long been separate from the land of the living, and in them we have a pledge of the resurrection of all. All that have ever lived upon earth will again be called to life, and render up their account: not one will be found missing. There is no such thing as annihilation. All that have ever fallen asleep in Christ will be found in his safekeeping: patriarchs, prophets, apostles, martyrs, down to the humblest servant of God in our own day. “Though unseen to us they all live to God.” “He is not the God of the dead, but of the living” (Luke 20:38). Their spirits live as surely as we live ourselves, and will appear hereafter in glorified bodies, as surely as Moses and Elijah on the mountain. These are indeed solemn thoughts! There is a resurrection, and men like Felix may well tremble. There is a resurrection, and men like Paul may well rejoice."
J. C. Ryle, Matthew, Crossway Classic Commentaries (Wheaton, IL: Crossway Books, 1993), 149–150.