William Barclay commented on Mt.16:28, "As Matthew records this phrase, it reads as if Jesus spoke as if he expected his own visible return in the lifetime of some of those who were listening to him. If Jesus said that he was mistaken. But we see the real meaning of what Jesus said when we turn to Mark's record of it. Mark has: And he said to them, "Truly, I say to you, there are some standing here who will not taste death before they see the Kingdom of God come with power" (Mk.9:1).
It is of the mighty working of his Kingdom that Jesus is speaking; and what he said came most divinely true. There were those standing there who saw the coming of Jesus in the coming, of the Spirit at the day of Pentecost. There were those who were to see Gentile and Jew swept into the Kingdom; they were to see the tide of the Christian message sweep across Asia Minor and cover Europe until it reached Rome. Well within the life-time of those who heard Jesus speak, the Kingdom came with power.
Again, this is to be taken closely with what goes before. Jesus warned his disciples that he must go to Jerusalem, and that there he must suffer many things and die. That was the shame; but the shame was not the end. After the Cross there came the Resurrection. The Cross was not to be the end; it was to be the beginning of the unleashing of that power which was to surge throughout the whole world. This is a promise to the disciples of Jesus Christ that nothing men can do can hinder the expansion of the Kingdom of God."