What does Luke 21:21 mean?
Jesus is describing to Peter, Andrew, James, and John (Mark 13:3) some of the horrors that Israel will face between His ascension and second coming. In this section, He gives more detail to His prophecy about the destruction of the temple (Luke 21:6) and describes how all of Jerusalem will fall. Here, He warns Jews to flee not just Jerusalem but to run to the mountains if they're in Judea; judgment is coming (Luke 21:22).In AD 67, after years of Jewish revolt against their Roman occupiers, General Vespasian and King Agrippa II joined forces and attacked Galilee. The Jewish survivors, particularly the Zealots who led the insurrections, fled to Jerusalem where they clashed with the more moderate residents, assassinated the Roman-friendly leadership, and started a civil war that lasted until Rome destroyed the city in AD 70.
When God sent the Babylonians to take the southern kingdom of Judah into exile, He told the Jews to submit: to surrender and go peacefully (Jeremiah 38:17–18). When the Roman armies surround Jerusalem (Luke 21:20), Jesus tells the people they must flee. Run to the mountains. Get out of the city. Do not return to join the fight, and certainly do not try to enter Jerusalem for the feasts. Abandon the Promised Land.
While the Romans surround Jerusalem, the Jews fight a civil war inside the city. In Jerusalem, the people starve. If they try to flee, the Romans enslave or kill them. Galilee is already taken. Judea isn't safe. Their only chance is to climb the mountains where, maybe, the soldiers won't want to follow. But even the outpost on Masada is taken three years later.