The "then" is referring to the context of verses 15-20; when you see the abomination of desolation, which Luke tells us is Jerusalem surrounded by armies. Now, we already saw that this happened in 67 AD when Cestius Gallus, the Roman general, laid siege to Jerusalem. The Great Tribulation is not an event yet future to us. It was "then," during the siege of Jerusalem by the Romans in the first century. This is made abundantly clear in the parallel text in Luke's gospel.
Luke 21:20-24 (NKJV) "But when you see Jerusalem surrounded by armies, then know that its desolation is near. Then let those who are in Judea flee to the mountains, let those who are in the midst of her depart, and let not those who are in the country enter her. For these are the days of vengeance, that all things which are written may be fulfilled. But woe to those who are pregnant and to those who are nursing babies in those days! For there will be great distress in the land and wrath upon this people. 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."
Notice who in particular verse 23 says the tribulation will come upon-- "the land", which is Jerusalem and "this people," which refers to the first century Jews, not the future world. Verse 24 gives us added details as to exactly what will happen in the Great Tribulation. We will look more closely at the details of verse 24 in a few moments. Right now I want us to examine:
Luke 21:22 (NKJV) "For these are the days of vengeance, that all things which are written may be fulfilled."
Luke tells us here that
ALL things which are written will be fulfilled in the destruction of Jerusalem. What does he mean by that? "All things which are written," refers to
prophecy.
All prophecy was to be fulfilled in the destruction of Jerusalem. Daniel tells us this very same thing in Daniel 9:24:
Daniel was told that 70 weeks had been determined on his people Israel, and the city of Jerusalem. By the end of this prophetic time period, God promised that six things would be accomplished. One of the things that Daniel was told would happen by the end of that period was that God would "seal up vision and prophecy". The Hebrew commentaries are in agreement on the meaning of to "seal up vision and prophecy" -- they say it means the end and complete fulfillment of all prophecy.
Daniel's prophecy, then, tells of the time when all prophecy would cease to be given and what had been given would be fulfilled. When would this be? Daniel's vision ends with the destruction of Jerusalem which we know occurred in 70 AD (Daniel 9:26):
So Luke is saying the same thing that Daniel said, which is that at the time Jerusalem is destroyed all prophecy will be fulfilled. What does that include? That would include the prophecy of the Second coming, the resurrection, the new heavens and earth, everything prophesied to Israel would be fulfilled at the time of Jerusalem's destruction.
The Great Tribulation (Matthew 24:21)
Daniel is about Antiochus IV and the Abomination of Tribulation is when he defiled the Temple by sacrificing a pig to Zeus.. Why don't you read it?