The Great Tribulation (KJV) or Great Distress (NIV) of Matthew 24:21, will be a time of persecution for believers in Christ who will not take the Mark of the Beast and worship his image. What will keep the people of God trusting during persecution? It will be our knowledge of the prophecies of the Word of God. There will be a heightened expectation of His coming to right the wrongs of injustice and evil that will be going on at the time. Those of the Church experiencing persecution will not be under any doubt as to what is happening in the world. The already fulfilled prophecies of Jesus, Paul, John, and the Old Testament prophets of events of the first three and a half years, plus the exact timing of the Abomination of Desolation happening right at the mid-point of the seven-year period will awaken many to the reality of the war between God and Satan. The facts of Bible prophecy fulfilled will bring many to Christ in that time frame. God will fulfill His promises to us. As believers are persecuted, they will hold in their hearts His precious promises that He is waiting to celebrate with His bride, and with each day passing, we will be closer to His coming. He said to His disciples:
“I tell you, I will not drink from this fruit of the vine from now on until that day when I drink it new with you in my Father's kingdom” (Matthew 26:29).
There is coming a time when Christ will come and take us to Himself in a marriage covenant. With each day that passes, those who are experiencing that difficult time will wonder if each day that comes is the day of His appearing, for the Great Tribulation period (Matthew 24:21) will not last the full 1,260 days, because for believers, “the elect,” it will be “cut short:”
“If those days had not been cut short, no one would survive, but for the sake of the elect those days will be shortened” (Matthew 24:22).
The elect, true believers in Christ, will be caught up (raptured) at some point known only to the Lord, but we do know that it will be during The Great Tribulation. What could be happening on earth to bring about a situation that if God did not step in, no one would survive as in the verse above? What will precipitate Christ’s coming?
25“There will be signs in the sun, moon and stars. On the earth, nations will be in anguish and perplexity at the roaring and tossing of the sea. 26People will faint from terror, apprehensive of what is coming on the world, for the heavenly bodies will be shaken. 27At that time they will see the Son of Man coming in a cloud with power and great glory. 28When these things begin to take place, stand up and lift up your heads, because your redemption is drawing near” (Luke 21:25-28, Emphasis mine).
The Lord Jesus comes for His people at that time (v. 27), after the cosmic signs in the sun, moon, and stars. “For as lightning that comes from the east is visible even in the west, so will be the coming of the Son of Man” (Matthew 24:27). The heavenly bodies themselves are shaken (v. 26). The Greek word translated shaken, saleuō, means to "waver, agitate, rock, topple and to shake.” When a man standing on earth looks to the heavens, it will seem as if the planetary systems themselves will be shaking, with stars falling from the sky. It appears that the earth will be tilting and moving on its axis for a time. Isaiah the prophet says the same thing:
19The earth is broken up, the earth is split asunder, the earth is thoroughly shaken. 20The earth reels like a drunkard, it sways like a hut in the wind (Isaiah 24:19-20).
No wonder that people will faint from terror, apprehensive of what is coming on the earth (v. 26). When the world is at its darkest hour, the Lord Jesus will come. “At that time they will see the Son of Man coming in a cloud with power and great glory” (Luke 21:27). Oh, how I long for that Day, and I hope you are too. Keith Thomas
 E-sword.com on the word translated "shaken."