How do Tribulation survivors become ready for the millennial kingdom?
Tribulation survivors become ready for the millennial kingdom by hearing the Gospel preached by the 144,000 and believing it. Those who receive that message become believers and enter the Kingdom; those who reject it are removed. Only believers, never unbelievers, go into the millennial kingdom.
Les teaches that the surviving people of the Tribulation will go into the Kingdom, but only believers enter. Among those survivors, a percentage will have heard the Gospel from the 144,000, become believers, and so be ready for the Kingdom. Survivors who rejected the Gospel from the 144,000 will be removed from the scene. He also notes the Jewish remnant fled Jerusalem at the midpoint to the mountains southeast, and the few survivors of Isaiah 24 survive the seven years. Jesus depicted this separation in Matthew 25, the sheep and the goats; that judgment involved Gentiles, with no Jews involved.