This generation shall not pass away, until all these things be accomplished. This is one of those prophecies which admit of a growing fulfillment. If the word "generation" ( γανεὰ) be understood to mean the sum total of those living at any time on the earth, the prediction would hold true as far as the destruction of Jerusalem was concerned.
The destruction of Jerusalem took place within the limits of the generation living in our Lord's time; and there might be some of those whom he was then addressing who would live to see the event. His prediction amounted, in fact, to this, that the destruction of Jerusalem would take place within forty years of the time when he was speaking.
But it may have a wider meaning. It may mean the Jewish people. Their city would be destroyed their power overthrown. They would be "peeled and scattered." But they would still remain a distinct and separate nation to the end of the world.
And there are other prophecies which show that with their national conversion to Christianity will be associated all that is most glorious in the future Church of God.