Why will the tribulation be so severe at this point?
The severity at this point comes from the simultaneous outpouring of Satan's wrath and God's wrath upon the whole human race, making it unlike anything the planet has experienced. This three-and-a-half-year period intensifies, while a remnant of Jews flees to the mountains under God's protection.
Les explains that although the entire seven-year period is tribulation, the point of Matthew 24:21 marks where things really get tough. He emphasizes that it is not only Satan's wrath but also God's wrath poured out on the whole human race, and what the planet will go through is mind-boggling. In connection, a remnant of Jews from every category of life will flee Jerusalem to the mountains, and God will watch over and protect them for the whole three and a half years.