This article describes the Great Tribulation written in Matthew 24:21.
Matthew 24:15-26
15 "Therefore when you see the 'abomination of desolation,' spoken of by Daniel the prophet, standing in the holy place" (whoever reads, let him understand),
16 "then let those who are in Judea flee to the mountains.
17 Let him who is on the housetop not go down to take anything out of his house.
18 And let him who is in the field not go back to get his clothes.
19 But woe to those who are pregnant and to those who are nursing babies in those days!
20 And pray that your flight may not be in winter or on the Sabbath.
21 For then there will be great tribulation, such as has not been since the beginning of the world until this time, no, nor ever shall be.
22 And unless those days were shortened, no flesh would be saved; but for the elect's sake those days will be shortened.
23 "Then if anyone says to you, 'Look, here is the Christ!' or 'There!' do not believe it.
24 For false christs and false prophets will rise and show great signs and wonders to deceive, if possible, even the elect.
25 See, I have told you beforehand.
26 "Therefore if they say to you, 'Look, He is in the desert!' do not go out; or 'Look, He is in the inner rooms!' do not believe it.
27 For as the lightning comes from the east and flashes to the west, so also will the coming of the Son of Man be.
Up until the time of Jesus, Jerusalem had been held captive since 604 BC by a succession of empires: Babylon, Persian, Greece, and Rome.
Matthew 24:15, The 'Abomination of desolation’ written in Daniel, depicts the Greek persecution of the occupants of Jerusalem during the time of this captivity.
When Jesus came, Jerusalem was still in captivity, but this time to Rome. Therefore, when Jesus said, 'when you see the ‘abomination of desolation,’ spoken of by Daniel the prophet,' Jesus was warning that Jerusalem's captivity was to continue. Just like when the Greeks persecuted Jerusalem, Rome was going to do the same thing. Luke chapter 21, the equivalent writing to Matthew 24, rather than talking about the 'abomination of desolation,' instead says, 'when you see Jerusalem surrounded by armies, then know that its desolation is near.' Jesus is warning the occupants of Jerusalem of a coming destruction by Rome that they are to flee from. That this is the time of vengeance, that Jerusalem will continue to be trampled down by the gentiles until their time is fulfilled and the occupants led away captive.
Luke 21:24
24 And they will fall by the edge of the sword and be led away captive into all nations. And Jerusalem will be trampled by Gentiles until the times of the Gentiles are fulfilled.
Matthew 24:21, At the time of Rome, Jerusalem was all that was left of the house of Israel. This assault on Jerusalem by Rome was only a portion of what Israel experienced. 720 Years before, the Assyrians had taken the majority of Israel captive leaving Jerusalem, only to be taken by Babylon years later. This is what the Bible calls ‘the great tribulation,’ or in Jeremiah, 'Jacob's trouble,' not a future judgement on the earth or the Church.
Matthew 24:24-26, The reason for Jesus' warning was because the Jews understood from the prophets that the Messiah was to come and restore the kingdom and save them from their enemies. But when the Messiah did come, they didn't recognise him, nor did they understand how He would restore the kingdom. Jesus explained that all things needed to be fulfilled first that was written in the prophets, that the tribulation had to go to completion. This is why Jesus warned of false prophets acting like the Messiah, who would lead people in a revolt against the Romans only to end in tragedy.
Matthew 24:27, Jesus goes on to explain, giving clues how the kingdom will be restored after the tribulation.