Why does the rapture (removal of the church) have to happen before the tribulation?
The church must be removed before the tribulation because God does not deal with Israel and the church at the same time. During the church age He deals primarily with Gentiles while Israel is dispersed; before He resumes dealing with Israel in the final seven years, the body of Christ must be taken out.
Les explains that this seven-year period was pushed into the future because Israel did not respond to the offer of the king and the kingdom. In between is the calling out of the body of Christ—the church age, the age of grace. God did not deal with the church when He was dealing with Israel; that was Jew only. Now He deals primarily with the Gentile while Israel is in dispersion. Therefore, before He can come back and deal with Israel again in the final seven years, the Gentile body of Christ has to be removed from the scene, because God does not deal on both sides of the fence at the same time. As the church age winds down and the final Gentile is brought in, the church is taken out, Israel is back in the land, and ready to be dealt with in the tribulation events.