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He was speaking literally in a way: "six days later", at the transfiguration, they did see some of the glory that Jesus would have at that time, and seeing Peter's reaction it was quite impressive. Read on in chapter 17:1-9, that's where your answer is.
In verses 10-13, Jesus explains a bit more about the time they lived in at that point.
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In addition to the answer above, below is a bible commentary:
Till they see the Son of man coming in his kingdom. Some expound this, as fulfilled at his transfiguration, which follows in the next chapter. Others understand it of the glory of Christ, and of his Church, after his resurrection and ascension, when he should be owned for Redeemer of the world: and this state of the Christian Church might be called the kingdom of Christ. (Witham)
This promise of a transitory view of his glory he makes, to prove that he should one day come in all the glory of his Father, to judge each man according to his works: not according to his mercy, or their faith, but according to their works. (St. Augustine, de verb. apos. serm. 35.)
Again, asks St. Augustine how could our Saviour reward every one according to his works, if there were no free will? (lib. ii. chap. 4. 5. 8, de act. cum FΕlic. Manich.) (Bristow)