What is the meaning of Matthew 24:21-22?

21 For then shall be great tribulation, such as was not since the beginning of the world to this time, no, nor ever shall be. 22 And except those days should be shortened, there should no flesh be saved: but for the elect’s sake those days shall be shortened. (Matthew 24:21-22 KJV)

William Burkitt’s Commentary

The doleful miseries and dreadful calamities which were coming upon the Jews in general, and upon Jerusalem in particular, are here foretold by our Saviour, partly from the Roman army without, and partly from the seditions and factions of the zealots within, who committed such outrages and slaughters, that there were no less than an hundred thousand slain, and ninety-seven thousand carried away captive, and made prisoners. They that bought our Saviour for thirty pence, were now themselves sold thirty for a penny. Now did the temple itself become a sacrifice, a whole burnt-offering, and was consumed to ashes.

Yet observe, Christ promises that these calamitous days shall be shortened for the elect’s sake. God had a remnant, which he determined should survive this destruction, to be an holy seed; and accordingly the providence of God so ordered, that the city was taken in six months, and the whole country depopulated in eighteen.

Whence observe, How the Lord intermixes some mercy with the extremest misery that doth befall a people for their sin. On this side hell, no sinners can say that they feel the strokes of justice to the utmost, or that they have judgment without mercy.