What does Mark 13:20 mean?
The first half of the tribulation will be horrible with war, disease, and persecution of those who come to Christ after the rapture. The last half, per a traditional interpretation, will be even worse. Over the course of these catastrophes, one-third of the vegetation will burn, all the sea creatures will die, a third of mankind will die in war, and a great earthquake will shake the world around the same time that hundred-pound hailstones fall to earth (Revelation 8:1–9:21; 16:1–21). At the end, nations will rebel against the Antichrist. All the armies will join to fight Jesus as He returns to earth (Matthew 24:30; Revelation 19:11–21). Jesus will destroy everyone who doesn't follow Him.God established in prophecy that the tribulation will last seven years and the great tribulation, or "wrath of God," will be the last three and a half years of this period (Daniel 9:27). This passage doesn't mean that He will cut those years short, but that He will halt the natural progression of the violence and terror before all His followers are destroyed. "Shortened" is from the Greek root word koloboo, and means to stop suddenly, to chop off. God will end the tribulation very quickly.
Those believers who survive the tribulation—"the elect"—will live in the millennial kingdom with resurrected or raptured believers who have received glorified bodies (1 Thessalonians 4:13–18; 1 Corinthians 15:21–23, 51–53; Revelation 20:4–6). The people with glorified bodies will neither die nor sin, but the survivors of the tribulation will be as mortal and human as we are now. They will marry, have families, and die. Although Jesus will reign from Jerusalem, the survivors' children and grandchildren will still have a choice whether to accept Him as their savior or not. After a thousand years, many of the descendants of these tribulation saints will have rejected Jesus. When Satan is released from the abyss, he will build an army to fight Jesus one more time. The short battle will result in Satan being thrown into the lake of fire forever (Revelation 20:10) and his human followers facing the great white throne judgment where they, too, will be sentenced to hell (Revelation 20:11–15).