Before “the end” of which Christ speaks arrived, the gospel of the kingdom would be preached in all the world.  I think “all the world” here would refer to the known world of that generation, which for the disciples would have largely been comprised of the Roman empire.  That “world” largely was covered by gospel evangelists before the Roman armies descended upon Jerusalem.  That this is in the context of the destruction of Jerusalem is proven by the next verses.  I know that many attempt to apply these verses to a great tribulation at the end of the world, but it seems evident to me that Christ is warning His disciples and fellow Jews of the great destruction that would befall their beloved capital at the hands of Rome.  It is certain that the early Christians took it this way, for because of this warning most, perhaps all of them, fled and escaped the wrath of Vespasian’s armies.  The unbelievers who had resisted Christ stayed to resist the Roman forces, and were swallowed up by an unspeakable calamity.

The time of Jerusalem’s destruction would be a time of great tribulation, such as the world had never seen, nor could ever be repeated.  The awful holocausts of Verdun or Stalingrad or Berlin could not compare to it.  Christ says that except God had shortened those days, no flesh should be saved.  It is very likely that He here intimates the entire Hebrew family would be annihilated, except that God chose to shorten the days that some of them might be saved, either then or at a future date.