In the following passages, I believe Jesus was referring to the Day of Pentecost:
(Matthew 16:28) – 28 Truly I tell you, there are some standing here who will not taste death until they see the Son of Man coming in his kingdom.” (CSB)
(Mark 9:1) – 1 Then he said to them, “Truly I tell you, there are some standing here who will not taste death until they see the kingdom of God come in power.” (CSB)
(Luke 9:27) – 27 Truly I tell you, there are some standing here who will not taste death until they see the kingdom of God.” (CSB)
Some believe that this was fulfilled in AD 70, with the destruction of Jerusalem and its temple. Others believe Jesus was referring to the Transfiguration (Matt 17:1-9; Mark 9:1-9; Luke 9:28-36). Others, like myself, believe it was fulfilled on the Day of Pentecost. To properly interpret this, we first need to identify who was present when Jesus made this statement. We don’t see everyone present in the Matthew or Luke passages (Matt 16:24-28; Luke 9:23-27), so we need to go to Mark:
(Mark 8:34-9:1) – 34 Then Jesus called the crowd, along with his disciples, and said to them, “If anyone wants to become my follower, he must deny himself, take up his cross, and follow me. 35 For whoever wants to save his life will lose it, but whoever loses his life for my sake and for the gospel will save it. 36 For what benefit is it for a person to gain the whole world, yet forfeit his life? 37 What can a person give in exchange for his life? 38 For if anyone is ashamed of me and my words in this adulterous and sinful generation, the Son of Man will also be ashamed of him when he comes in the glory of his Father with the holy angels.” 9:1 And he said to them, “I tell you the truth, there are some standing here who will not experience death before they see the kingdom of God come with power.” (NET)
It was not just the twelve disciples who were present. There was also a “crowd” of people, and most likely a very large crowd, as Jesus always attracted a large number of people. Within that crowd, there were probably many believers. The knowledge of a crowd of people present with Jesus, enables us to interpret what He said with confidence. When He said “there are some standing here who will not taste death until they see the kingdom of God come in power,” it was out of that crowd of people that He was referring to — more specifically, among those who believed.
Therefore, what Jesus was actually revealing is that there were only a few people among them who would actually “see” the Kingdom of God “coming” — as happened on the Day of Pentecost, before they died. The rest of those who believed among the crowd, who were not present at Pentecost, they did not “see” the “coming” of God’s kingdom. While we all as believers “experience” the Kingdom of God after Pentecost ‒ having received the Holy Spirit and baptized into the body of Christ (the Church) ‒ only a few of the believers of that day actually witnessed the “coming” of His kingdom on that day (“with power”), as Acts chapter two describes. The rest of the believers who were in the crowd, died without seeing the coming of God’s kingdom at Pentecost.
The Kingdom of God is a spiritual kingdom, which is the Church. More specifically, the Church is the Kingdom of Christ (Matt 16:28; Col 1:13). The Church as we know it and experience it, began on the day of Pentecost. Only a small number of believers actually witnessed the initial coming of it on that day (about 120 people). The rest of those who believed in Jesus’ day, died without seeing it. In other words, they weren’t present to witness all that transpired on that day.