Mark 13:20

And except the Lord had shortened the days, no flesh would have been saved; but for the elect's sake, whom he chose, he shortened the days.

The elect's sake ... These were Christians, "whom he chose," not through some immutable decree, or capricious election before all time and eternity, but through the gospel. God chose and elected the people who would receive and adore the Christ, obey his gospel, and accept God's forgiveness. No one was excluded by such an election, each man deciding for himself whether he would come into the community of "the elect" or continue with the rebellious.

He shortened the days ... The use of the past tense, both by Christ and by Mark, is prophetic, speaking of that which God has decreed for the future as being already done. How this shortening was accomplished is not exactly known. Sanner wrote, "Impelled by matters of pressing personal concern, the Roman generals hastened back to Italy."[42] Some of the events that might have entered into their breaking off of the Jewish war without the total destruction of the whole nation were: (1) disturbances in Gaul that interfered with the campaign of Vespasian; (2) the death of Nero plunged Rome into war as Otho and Vitellius each sought to be emperor; (3) Vespasian was declared emperor by his soldiers, their verdict being final, due to the might of the military; (4) Vespasian returned to Rome as emperor, and (5) his son Titus concluded the siege.