Why can no unbelievers go into the kingdom?
No unbelievers can enter the kingdom because Satan will be locked up, removing his influence, and the kingdom will begin with a generation of parents who are believers. This ensures the kingdom starts with a believing population, as described in Matthew 25 after the tribulation.
Les explains that after the final bowl judgment, there will be survivors in every nation—saved and lost. The 144,000 Jewish witnesses will proclaim the King and the kingdom at hand, and salvation is always based on the cross. Then, at the judgment in Matthew 25, no unbelievers can enter the kingdom because Satan is locked up and the kingdom begins with a generation of parents who are believers. This prevents unbelievers from corrupting the kingdom's start.