Jesus heals a blind man then asks His disciples who they think he is; Peter says He is the Messiah; Jesus then explains how He will soon die and be resurrected but Peter objects, only to be told by Jesus that giving up your own selfish wants was part of following Him: Mark 8:22–30; Matthew 16:13–20; Luke 9:18–20; Mark 8:31–9:1; Matthew 16:21–28; Luke 9:21–27; Mark 9:2–13; Matthew 17:1–13; Luke 9:28–36
It’s easy to see how confusing a statement like this could be:
Mark 9:1 I tell you the truth, some standing here right now will not die before they see the Kingdom of God arrive in great power!
How easy would it be to assume, as the Disciples might have, that this would be a physical arrival of the Kingdom of God sometime before they had all died? It would meet community expectations of a Messiah that would fight to restore Israel.
But Jesus kept saying the Kingdom of Heaven was already there, because He was there:
Luke 17:20-21 The Kingdom of God can’t be detected by visible signs. You won’t be able to say, ‘Here it is!’ or ‘It’s over there!’ For the Kingdom of God is already among you.
And there was no shortage of God’s power being demonstrated when Jesus was around. But the text suggests some future event.
For that, Jesus’ resurrection is an obvious candidate: there was power, even angels, and it really marked the defeat of death.
More likely is the day of Pentecost. It was an obvious arrival of the Kingdom of God arriving with power, and most of the disciples were still alive, and they were there:
Acts 2:1-4 On the day of Pentecost all the believers were meeting together in one place. Suddenly, there was a sound from heaven like the roaring of a mighty windstorm, and it filled the house where they were sitting. Then, what looked like flames or tongues of fire appeared and settled on each of them. And everyone present was filled with the Holy Spirit and began speaking in other languages, as the Holy Spirit gave them this ability.
And this event changed the way believers lived their lives. It was the arrival of the Kingdom of God with great power.