We can see the kingdom of God

Luke
Jesus said in Luke 9:27 that there would be some among the disciples who would not taste death until they saw the kingdom of God.  What does this mean?  The kingdom of God sometimes seems to be a bit ethereal, not just in the sense that it is part of the spiritual world, but also in that it seems like we could never be a part of it until we die.  But that's not the case.
The kingdom of God exists wherever God has sovereign control.  In one sense, that means everything is God's kingdom.  However, I think that in this context, and in most places where this phraseology is used in the bible, the kingdom of God refers to wherever people are joining in God's mission and working to extend his power in this earthly realm.  We can do this through the Holy Spirit.  Thus, God's kingdom probably truly started at Pentecost when the Holy Spirit came on the disciples.
I'm getting a bit of help from Gill's Exposition on the Bible for this idea, but, in Luke 9:27, when Jesus said some of the disciples wouldn't die until they saw the kingdom of God, he meant that some of them would survive until he was raised to life and they could see firsthand his true power and authority as God's Son.