What is the "Great Tribulation" of Matthew 24:21

The word tribulation was used by Jesus and the apostles more than twenty times in the New Testament to refer to the distress and trials that Christ’s followers must suffer in this life.  For example, Acts 14:22 tells us that Paul and Barnabas taught that we should “continue in the faith, and that we must through much tribulation enter into the kingdom of God. The words “great tribulation” found in Matthew 24:21 describe a period of unparalleled suffering. “For then shall be great tribulation, such as was not since the beginning of the world to this time, no, nor ever shall be.”  Premillennialism incorrectly teaches that the “great tribulation” is a seven-year period in the future that follows the “rapture” of the church and precedes the Lord’s second coming. (There are variations among premillennialists, but these ideas are generally accepted.)
 
The disciples asked Jesus privately, “Tell us, when shall these things be? and what shall be the sign of thy coming, and of the end of the world?” (Matthew 24:3). Notice they asked about two things: “the sign of thy coming,” and “the end of the world”. These two separate things are the destruction of Jerusalem, and the Lord’s second coming at the end of the world.  Chapters 24 and 25 of Matthew give Christ’s description of these two events.  Matthew 24:1-35 describes the destruction of Jerusalem that occurred in 70 A.D., while the remainder of the passage describes the Lord’s second coming.
 
The great tribulation, which Premillennialism wants to project to the end of time, is a description of the destruction of Jerusalem. Jesus said, “This generation shall not pass, till all these things be fulfilled” (Matthew 24:34).  Therefore, within a Biblical generation (thirty to forty years), the events Christ described would come to pass. The destruction of Jerusalem in 70 A.D. fulfilled this prophecy. Daniel had prophesied a coming “abomination that maketh desolate” (Daniel 11:31; 12:11). Jesus stated “When ye therefore shall see the abomination of desolation, spoken of by Daniel the prophet, stand in the holy place, (whoso readeth, let him understand)” (Matthew 24:15).  The abomination of desolation was the desecration and destruction of the temple by the Romans as they destroyed Jerusalem.
 
The fact that this great tribulation was the destruction of Jerusalem, and not a coming period of misery, is demonstrated by Jesus’ instructions to His followers to watch for the signs, and “flee into the mountains” to escape, and saying they should pray that it “be not in the winter,” when the weather would be bad, nor that their flight be “on the Sabbath day” when the city gates would be closed (Matthew 24:16, 20). History shows that Christians understood this warning and were able to escape the destruction of Jerusalem by fleeing.  Eusebius the historian reveals that Christ’s prophecies were realized and the believers obeyed the warnings, fled Jerusalem to a town called Pella (and others), and saved themselves. Eusebius wrote: “The whole body, however, of the church at Jerusalem, having been commanded by a divine revelation, given to men of approved piety there before the war, removed from the city, and dwelt at a certain town beyond the Jordan, called Pella.”  All who follow Christ will suffer tribulation, but the period of great tribulation has already passed.