Why can't the church be mixed up with Israel in the tribulation?
The church cannot be mixed with Israel in the tribulation because that period is God's dealing with the nation of Israel, not the church. Israel remains God's chosen nation, and Scripture insists He will not cast off the seed of Israel; the tribulation is primarily His dealing with Israel.
Les teaches that the tribulation cannot be a time when the church is mixed up with Israel, because this is God's dealing with Israel, not the church. He points to Jeremiah 31, where God's covenant promises to Abraham, Isaac, and Jacob still stand. In Jeremiah 31:35-36, God says that if the ordinances of the sun, moon, and stars depart from before Him, then the seed of Israel would cease from being a nation forever. Since those laws of nature continue, Israel is not cast aside. Les explains that even though Israel rejected and crucified their Messiah, God raised up Paul and sent him primarily to the Gentiles. Individual Jews can still be saved in the age of grace, but the tribulation itself is primarily God dealing with the nation of Israel, not the church.