What was the transfiguration?

The Bible describes a miraculous event in Christ's ministry often called the transfiguration of Jesus. What was it?

The transfiguration is recorded for us in Matthew 17:1-13 [1] And after six days Jesus taketh Peter, James, and John his brother, and bringeth them up into an high mountain apart,
[2] And was transfigured before them: and his face did shine as the sun, and his raiment was white as the light.
[3] And, behold, there appeared unto them Moses and Elias talking with him.
[4] Then answered Peter, and said unto Jesus, Lord, it is good for us to be here: if thou wilt, let us make here three tabernacles; one for thee, and one for Moses, and one for Elias.
[5] While he yet spake, behold, a bright cloud overshadowed them: and behold a voice out of the cloud, which said, This is my beloved Son, in whom I am well pleased; hear ye him.
[6] And when the disciples heard it, they fell on their face, and were sore afraid.
[7] And Jesus came and touched them, and said, Arise, and be not afraid.
[8] And when they had lifted up their eyes, they saw no man, save Jesus only.
[9] And as they came down from the mountain, Jesus charged them, saying, Tell the vision to no man, until the Son of man be risen again from the dead.
[10] And his disciples asked him, saying, Why then say the scribes that Elias must first come?
[11] And Jesus answered and said unto them, Elias truly shall first come, and restore all things.
[12] But I say unto you, That Elias is come already, and they knew him not, but have done unto him whatsoever they listed. Likewise shall also the Son of man suffer of them.
[13] Then the disciples understood that he spake unto them of John the Baptist.

See All..., Mark 9:2-13 [2] And after six days Jesus taketh with him Peter, and James, and John, and leadeth them up into an high mountain apart by themselves: and he was transfigured before them.
[3] And his raiment became shining, exceeding white as snow; so as no fuller on earth can white them.
[4] And there appeared unto them Elias with Moses: and they were talking with Jesus.
[5] And Peter answered and said to Jesus, Master, it is good for us to be here: and let us make three tabernacles; one for thee, and one for Moses, and one for Elias.
[6] For he wist not what to say; for they were sore afraid.
[7] And there was a cloud that overshadowed them: and a voice came out of the cloud, saying, This is my beloved Son: hear him.
[8] And suddenly, when they had looked round about, they saw no man any more, save Jesus only with themselves.
[9] And as they came down from the mountain, he charged them that they should tell no man what things they had seen, till the Son of man were risen from the dead.
[10] And they kept that saying with themselves, questioning one with another what the rising from the dead should mean.
[11] And they asked him, saying, Why say the scribes that Elias must first come?
[12] And he answered and told them, Elias verily cometh first, and restoreth all things; and how it is written of the Son of man, that he must suffer many things, and be set at nought.
[13] But I say unto you, That Elias is indeed come, and they have done unto him whatsoever they listed, as it is written of him.

See All... and Luke 9:28-36 [28] And it came to pass about an eight days after these sayings, he took Peter and John and James, and went up into a mountain to pray.
[29] And as he prayed, the fashion of his countenance was altered, and his raiment was white and glistering.
[30] And, behold, there talked with him two men, which were Moses and Elias:
[31] Who appeared in glory, and spake of his decease which he should accomplish at Jerusalem.
[32] But Peter and they that were with him were heavy with sleep: and when they were awake, they saw his glory, and the two men that stood with him.
[33] And it came to pass, as they departed from him, Peter said unto Jesus, Master, it is good for us to be here: and let us make three tabernacles; one for thee, and one for Moses, and one for Elias: not knowing what he said.
[34] While he thus spake, there came a cloud, and overshadowed them: and they feared as they entered into the cloud.
[35] And there came a voice out of the cloud, saying, This is my beloved Son: hear him.
[36] And when the voice was past, Jesus was found alone. And they kept it close, and told no man in those days any of those things which they had seen.

See All.... The preserved accounts tell us that Jesus took three of His disciples high on top of a mountain, where they saw Him "transfigured before them. His face shone like the sun, and His clothes became as white as the light" (Matthew 17:2And was transfigured before them: and his face did shine as the sun, and his raiment was white as the light.
See All...). He then appeared to be having a conversation with Moses and Elijah. Luke records that they "appeared in glory and spoke of His decease which He was about to accomplish at Jerusalem" (Luke 9:31Who appeared in glory, and spake of his decease which he should accomplish at Jerusalem.
See All...).

Jesus told Peter, James and John to "tell the vision to no one until the Son of Man is risen from the dead" (Matthew 17:9And as they came down from the mountain, Jesus charged them, saying, Tell the vision to no man, until the Son of man be risen again from the dead.
See All..., emphasis added). So the disciples saw a vision of the future, not something that was actually happening in their time.

We can also see this in the phrase "appeared in glory." The apostle Paul writes that at death a Christian's body is "sown in dishonor, it is raised in glory" (1 Corinthians 15:43It is sown in dishonour; it is raised in glory: it is sown in weakness; it is raised in power:
See All...). When Christ's followers are resurrected at Christ's return to earth, they will be "raised in glory." However, Christ's resurrection to His glorified form made Him the "firstborn from the dead" (Revelation 1:5And from Jesus Christ, who is the faithful witness, and the first begotten of the dead, and the prince of the kings of the earth. Unto him that loved us, and washed us from our sins in his own blood,
See All...). Since Christ is first, it is impossible that Moses and Elijah were resurrected to glory before Christ. This helps show that the transfiguration was only a vision of things to come.

In a similar fashion to the book of Revelation, where the apostle John was given a vision of end-time events as if they were happening around him, the transfiguration gave Peter, James and John a partial vision of the Kingdom of God as if it were truly there.

Christ had promised this vision six to eight days prior in Luke 9:27But I tell you of a truth, there be some standing here, which shall not taste of death, till they see the kingdom of God.
See All..., where He told a crowd (which included Peter, James and John) that "there are some standing here who shall not taste death till they see the kingdom of God" (see also Matthew 16:28Verily I say unto you, There be some standing here, which shall not taste of death, till they see the Son of man coming in his kingdom.
See All... and Mark 9:1And 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.
See All...). The Kingdom of God is not yet here, but everyone in that 2,000-year-old crowd has "tasted death." Christ's statement was fulfilled when three of His apostles witnessed the transfiguration on the mount, a vision of that same Kingdom.