Luke 21:21

“Then let those who are in Judaea flee to the mountains, and let those who are in the midst of her depart out, and let not those who are in the countryside enter into it.”

The warning here is vivid and pointed. At the first indication of approaching troops they are to find refuge not in the city (the natural place of refuge in time of war), for that is doomed, but in the caves on the wild, deserted mountains. That what is being described here is local can be seen from the fact that it can be avoided by fleeing to the mountains in the vicinity of Judea, a flight also to be engaged in by those in Jerusalem with all speed. Nor are those in the countryside to see Jerusalem as a refuge. The emphasis is on the fact that the judgment is centred on Jerusalem, and is certain, although necessarily it will involve all concerned with the welfare of Jerusalem. In the event the whole of Galilee and Judea would be affected, which was ever the case when Jerusalem was to be invested as past investments had made clear (when Sennacherib invested Jerusalem he had besieged and taken forty six large cities. Nebuchadnezzar had engaged in wholesale destruction). But Jerusalem would experience the total devastation, for after huge slaughter of both young and old, male and female, the remainder were carried off into captivity. No mercy was shown by the Roman invaders.

For this idea of fleeing to the mountains see Ezekiel 7:16; Genesis 19:26; Isaiah 15:5; and compare also Jeremiah 49:8; Zechariah 14:5; Amos 5:18-20. As Jerusalem is itself in the mountains the idea is of the remote mountains of the Judean wilderness in the Dead Sea area, to which David fled to escape from the vengeance of Saul (1 Samuel 26:1-3), as well as the mountains across the Jordan in Transjordan.