Was Jesus' offer of the Kingdom to Israel a real and valid offer?

Yes. When Christ came in His earthly ministry, He made a valid offer to be Israel's King and to set up the Kingdom. It was a real offer; He was not playing games. Even if Israel had accepted, God would have brought about the crucifixion one way or another, because it had to happen.

Les teaches that when Christ came in His earthly ministry, He made a valid offer to be Israel's King and to set up the Kingdom. It was a valid offer; He wasn't playing games. Les then addresses the hypothetical question: what if Israel had embraced Him as their King and He set up the Kingdom before the crucifixion? He answers that with God nothing is impossible. Had Israel accepted the Kingdom, God would have brought about the crucifixion one way or another, because it had to happen. Christ had to die that sacrificial death, and there was no escaping that.

Psalms 2:1

Why do the heathen rage, and the people imagine a vain thing?

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What would have happened if Israel had accepted Jesus as King before His crucifixion?

If Israel had accepted Christ as King and the Kingdom had been set up before the crucifixion, the crucifixion still had to happen. God would have brought it about one way or another, because Christ had to die that sacrificial death, and there was no escaping it.