Matthew 24:21

“For then will be great tribulation, such as has not been from the beginning of the world until now, no, nor ever will be.”

And the reason for their flight would be so as to avoid ‘great tribulation and suffering' that would come on those who remained behind. First there would be the unbelievable intensity of the suffering of the siege (the story of what happened in the city is almost incredible) combined with the devastation caused by the besieging army to the surrounding area, and this would be followed by the appalling treatment meted out to the besieged once the siege was over, with many being crucified and large numbers being forced into a long, unceasing exile, from which they would never return. Luke describes it graphically, ‘they will be led captive into all the nations, and Jerusalem will be trodden down of the Gentiles until the time of the Gentiles be fulfilled' (Luke 21:24). Thus the great tribulation would extend into the unknown future, as graphically described in Deuteronomy 28:49, and including the whole miserable history of the Jews. It would be such that none other would ever suffer the like again. Note the ‘nor ever will be', which indicates a considerable time gap following the initial commencement of the tribulation. A long period of time was expected to follow the first initial experiences of this event, and it is true that no other nations have suffered throughout their history like the Jews. (This is in contrast with the time of trouble in Daniel 12:1 where the time of trouble described there does not end with the words ‘nor ever will be'. It is therefore referring to a different time of trouble).

Combining the three accounts in the Gospels we would come up with the following:

“But woe to those who are with child and to those who are breast feeding in those days!  “And pray that your flight be not in the winter, nor on a sabbath,”  “For then will be great tribulation, such as has not been from the beginning of the creation which God created, until now, no, nor ever shall be, for there will be great distress on the land, and wrath to this people.”  “And they will fall by the edge of the sword, and will be led captive into all the nations, and Jerusalem will be trodden down of the Gentiles, until the times of the Gentiles be fulfilled.  “And except those days had been shortened, no flesh would have been saved, but for the elect's sake those days will be shortened.”