What does Luke 21:20 mean?

Jesus mentions, briefly, that the grand buildings of the temple will be destroyed until no stone stands upon another (Luke 21:6). The disciples ask what signs will precede this tragedy (Luke 21:7). He responds with an overview of the false teachers, wars, and natural and supernatural disasters that will typify the time between the temple's destruction and His return (Luke 21:8–11). He then describes three events: the persecution of His followers (Luke 21:12–19), the destruction of Jerusalem (Luke 21:20–24), and the return of the Son of Man (Luke 21:25–28). Although the world will be impacted (Luke 21:35), these events are focused on the Jews.

Now, He answers their question. The sign that will precede the destruction of the temple is that armies will surround Jerusalem. But the entire city, not just the temple, will be destroyed. He has already given the reason this will happen: because the Jews as a nation rejected their Messiah (Luke 19:43–44).

Jerusalem fell in AD 70 at the end of a long chain of escalating incidents. Except for the elders and Sadducees who financially benefited, the Jews of Jesus' time did not appreciate the Roman occupation. Some, the Zealots, fought against it. Insurrections regularly popped up and the Romans used varying degrees of violence to squash them.

In AD 66, things got serious. The Romans in Jerusalem had steadily increased their disrespect of the temple. The emperors had let the Jews worship their God with relative autonomy, which was unusual in the Empire, but persecution grew—as did taxes. In AD 66, the Roman procurator Gessius Florus stole silver from the temple treasury. A large group of Jews first took over the garrison in Jerusalem and then defeated Roman reinforcements from Syria. Confidence in the Zealots' ability to rout the Romans grew.

Their confidence was misplaced. General Vespasian, aided by King Agrippa II, attacked Galilee in AD 67, and the survivors fled to Jerusalem. The fleeing Zealots killed the Jewish rulers who did not support their cause, starting a civil war inside the city. Vespasian left for Rome to become emperor but gave the war to his son Titus and his trusted official Tiberius. They put up siegeworks around Jerusalem and let the Jews fight amongst themselves. The Roman army breached the walls in AD 70 and burned the temple. Between the civil war and the Romans, Josephus recorded that over a million Jews were killed and nearly a hundred thousand were taken captive.

There's a debate as to whether Jesus' prophecy refers to AD 70 or the attack of the Antichrist in our future. Matthew 24:15–22 and Mark 13:14–20 seem to add more detail, like a repeat of the abomination of desolation, prophesied by Daniel (Daniel 9:27; 11:31; 12:11) and first fulfilled in 167 BC by Antiochus IV and to be repeated by the Antichrist.

It seems that Matthew and Mark record a prophecy with double fulfillment. They talk about a "great tribulation, such as has not been from the beginning of the world" (Matthew 24:21) and that "if those days had not been cut short, no human being would be saved" (Matthew 24:22). That obviously doesn't apply to the events in the First Century but to the tribulation. Luke, on the other hand, ends Jesus' prophecy with "Jerusalem will be trampled underfoot by the Gentiles, until the times of the Gentiles are fulfilled" (Luke 21:24). The "times of the Gentiles" are between AD 70 and the tribulation. Luke more carefully differentiates between the fall of Jerusalem and the tribulation.

Jesus mentions the Roman armies surrounding the city. Between the civil war and the Romans, Jerusalem was not safe for anyone. Jesus warns those in Judea to flee to the mountains (Luke 21:21). Many survivors fled to Masada, a high, small plateau above Jerusalem, but the Romans took that refuge three years later.