In the story we call the transfiguration on the mountain, Jesus takes Peter, James and John up a mountain. The Lord is transfigured before their eyes (Greek: metamorphoo), His face shines like the sun and His outer garment becomes white as the light. They also see Moses and Elijah, who speak with Him (Matt. 17:1-3).
vision
How can Moses and Elijah appear to the disciples if they are dead? The answer is that Moses and Elijah were seen by the three disciples, by means of a vision (Matt. 17:9).
The same word (Greek: horama) is used in Acts 12:9, where Peter is imprisoned, chained between two guards. An angel appears and the chains fall from his hands, after which he walks out of the prison. The commentary on this is telling.
Acts 12
10 (…) And he did not know that it was a reality which was done by the angel, but he thought he saw a vision.
reality and vision
What happened here was reality and not a vision. At the transfiguration on the mountain it is the other way around. There it is a vision and not reality. The three disciples have been given a glimpse into the future, a preview of the Lord in His Kingdom. In the last verse of Matthew 16 Jesus says:
Matthew 16
28 Amen! I say to you: There are some of those standing here who will not taste death until they see the Son of Man coming in His Kingdom.
the seventh day
Then it says in the next verse that after six days the Lord takes them up to the mountain (Matt. 17:1). After six days, on the seventh day, the disciples receive this vision. The symbolism is very clear here. A mountain is a high place and a representation of the Kingdom. And it takes place on the seventh day, a Sabbath. The seventh day of a thousand years (2 Pet. 3:8), the Great Sabbath will dawn, the Kingdom of the Messiah.
On the seventh day the disciples receive a vision of the Lord in His Kingdom and Moses and Elijah appear, as the representatives of the law and the prophets.
What Peter says about this event in his second letter is also special.
2 Peter 1
16 For we have not followed wisely devised fables when we make known to you the power and presence (Greek: parousia) of our Lord Jesus Christ, but have become eyewitnesses of his majesty.
parousia
Peter says that he was an eyewitness of the parousia of Christ. Parousia means presence and is usually translated in the current translations as (second) coming. Peter was allowed to see things that are still future to this day and he was transferred, just like John in Revelation 1:10, to the day of the Lord. God confirmed the prophetic word to Peter (2 Pet. 1:19) and through him He does that to us.
If Moses and Elijah are not dead, but were already glorified, then Christ is not the only One who has immortality (1 Tim. 6:16) and He is also not the Firstfruits (1 Cor. 15:20, 23).