The promise is followed by a mountain
Jesus told those around him that some would see the kingdom of God before death. Matthew speaks of seeing the Son of man coming in his kingdom, while Mark speaks of the kingdom coming with power (Luke 9:27; Matthew 16:28; Mark 9:1). Taken by itself, the saying can sound like a prediction that the final return would happen during the disciples' lives.
But each Gospel places the Transfiguration directly after it. Within about a week, Jesus took Peter, James, and John up a mountain, where his appearance changed before them in glory (Luke 9:28-29; Mark 9:2-3). Some, rather than all, of those listening saw this extraordinary sight. That close connection gives the saying its most immediate explanation.
But I tell you of a truth, there be some standing here, which shall not taste of death, till they see the kingdom of God.
A preview, not the final day
On the mountain, Moses and Elijah appeared with Jesus, and the disciples saw his heavenly splendour (Luke 9:30-34). Then the voice from the cloud identified Jesus as God's chosen Son and called the disciples to listen to him (Luke 9:35). This was not the final judgement or the public renewal of the world. It was a glimpse behind the veil, showing the royal glory that truly belonged to Jesus.
Peter himself later connected that mountain experience with Christ's power and coming. He insisted that he, James, and John had seen Jesus' majesty with their own eyes, rather than following an invented tale (2 Peter 1:16-18). His reflection strongly supports reading the Transfiguration as a foretaste of Christ's coming kingdom.
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. And his raiment became shining, exceeding white as snow; so as no fuller on earth can white them.
The kingdom has a present and future side
Jesus' kingdom was already breaking into the world through his ministry, death, resurrection, and risen authority. After rising from the dead, he told his disciples that all authority in heaven and earth had been given to him, then sent them out to make disciples (Matthew 28:18-20).
Yet Christians also look forward to Jesus appearing openly in glory. Jesus spoke of a future time when people would see the Son of man coming with power and great glory (Luke 21:27). The Transfiguration does not replace that hope. It gives a brief, brilliant preview of it.
So was Jesus mistaken?
No. Peter, James, and John saw Jesus transfigured only days after the promise. They witnessed the kingdom's power and the Son's majesty before their deaths, which fits Jesus' careful wording.
Faithful Christians differ over whether the saying may also echo later events such as the resurrection, Pentecost, or Jerusalem's fall. Those events certainly displayed Christ's authority. Still, the Transfiguration is the clearest first fulfilment because all three Gospel accounts place it immediately after Jesus' words. Reading the whole scene together helps the difficult sentence make sense.