V.27-28 For the Son of Man is going to come in his Father's glory with his angels, and then he will reward each person according to what he has done. I tell you the truth, some who are standing here will not taste death before they see the Son of Man coming in his kingdom."

First it says they will see the Son of man coming in his kingdom before they died. They did. The next chapter continues:

Matt 17:1 Now after six days Jesus took Peter, James, and John his brother, led them up on a high mountain by themselves.”

In Mark 9:1-10: And He said to them, “Assuredly, I say to you that there are some standing here who will not taste death till they see the kingdom of God present with power.” Now after six days Jesus took Peter, James, and John, and led them up on a high mountain apart by themselves; and He was transfigured before them.

Also Luke 9:27-37: “But I tell you truly, there are some standing here who shall not taste death till they see the kingdom of God.”  Now it came to pass, about eight days after these sayings, that He took Peter, John, and James and went up on the mountain to pray”.

So how did they see this? They saw the glory of His kingdom on the Mt. of Transfiguration. Two accounts Matthew and Mark say 6 day's, but Luke says 8 day's. Jesus often spoke of something that was important more than once- that is why we have recorded 8 days and also 6 days  for the same event, he said it more than once.

Mk.9:2 “He was transfigured before them- the light came from within him not from without showing the glory that was hidden from them since his birth. This is why Jesus is called “the brightness of His glory and the express image of His person” (Heb 1:3) he forever is “the Lord of glory” (1 Cor .2:8). When John sees Jesus in heaven he is enthroned, and seen in his glory revealed. Rev 1:16 “His countenance was like the sun shining in its strength.

While Jesus is PRAYING he is transfigured; and Moses and Elijah both appear in glory (Lk.9). Matthew states Jesus’ face shone like the sun, and His clothes became as white as the light, Mark reports He was transfigured that his clothes became shining, exceedingly white, like snow, such as no launderer on earth can whiten them . Luke says the appearance of His face was altered, and His robe became white and glistening. His glory showed through his human nature, even his clothes.

Mt.17 “While he was still speaking, behold, a bright cloud overshadowed them; and suddenly a voice came out of the cloud, saying, “This is My beloved Son, in whom I am well pleased. Hear Him!” If they were not there for his baptism when God spoke from heaven and the Spirit came upon him like a dove, they were here experiencing something more spectacular. Just as a cloud covered the mountain and the glory of the LORD rested on Mount Sinai, and the cloud covered it six days (Exod. 24:15-18) Moses once again is on a Mountain engulfed in the cloud and hearing God’s voice from heaven. This too is called a holy mountain by Peter.

The presence of Moses and Elijah are associated with the kingdom, they were expected by the Jewish people to return at the end of the age with the Messiah. This is why Peter wanted to build them tabernacles. He thought the kingdom had arrived with their being present and seeing Jesus glorified.

The three closest disciples were given a preview of what it would be like in the kingdom, this is how Peter describes it in 2 Pt. 1:16-18 “ For we have not followed cunningly devised fables, when we made known unto you the power and coming (parousia) of our Lord Jesus Christ, but were eyewitnesses of his majesty. For he received from God the Father honor and glory, when there came such a voice to him from the excellent glory, This is my beloved Son, in whom I am well pleased. And we heard this voice which came from heaven when we were with him on the Holy mountain.”

Jesus said previously, some would not die until they see the coming of the kingdom in power.” They did even though it was temporary.

Jesus needs to return to establish the kingdom on earth permanently. This encompasses His physical return with angels and glory that will be seen by all.

Mt16:27 For the Son of Man is going to come in his Father's glory with his angels, and then he will reward each person according to what he has done” (Mt.25)

Mark 13:24-27: V.24 "But in those days, after that tribulation, the sun will be darkened, and the moon will not give its light; (this is describing the tribulation period commencing Rev.6:12, Joel 2:10,31).

v.26 "Then they will see the Son of Man coming in the clouds with great power and glory."And then He will send His angels, and gather together His elect from the four winds”

So we have Jesus coming back to earth in glory, with angels to have his kingdom in power on earth.

No doubt John had this event in mind along with witnessing his whole life. “And the Word became flesh and dwelt among us, and we beheld His glory, the glory as of the only begotten of the Father, full of grace and truth.”(John 1:14)