What does Mark 9:3 mean?
In Mark 9:1, Jesus promises that some of the disciples will see "the kingdom of God after it has come with power" before they die. This, according to many interpreters, is the display of the power of the kingdom of God Jesus promised. We aren't told specifically why Peter, James, and John are chosen. Peter, perhaps, because of the rough road he will continue to take as the Holy Spirit works His sanctifying power in him as he becomes a great leader in the church (Galatians 2:9). John, maybe, for his significant theological writings that have educated believers for the last two thousand years. As for James, he was the first of the Twelve to be martyred (Acts 12:2). The honor of seeing the transfiguration may have bolstered his faith as he metaphorically met the cross Jesus spoke of (Mark 8:34–38).Matthew 17:2 says His clothes became as "white as light;" Luke 9:29 says "dazzling white," like a flash of lightning. The original text doesn't say "bleach" but "whiten." Although the English Standard Version says no "one," here, other versions give the more precise word of "launderer" or "fuller." A fuller is someone who uses nitrium to clean woolen cloth. In heaven, the twenty-four elders and the tribulation martyrs will also receive white robes (Revelation 4:4; 7:13–14).
Jesus' clothes are a metaphor for the righteousness that characterizes the presence of God. We will never be good enough, just as a launderer can never bleach Jesus' robes white enough. In order to be acceptable to God, we must accept Jesus' gift of forgiveness of our sins. This is why He came, and this is the message that the disciples have trouble understanding.