What will Israel face during the tribulation?
During the tribulation, Israel will face the greatest discipline ever experienced by any people, as God disciplines them for their unbelief and sin. However, He will not destroy the nation or turn His back on them forever; He has set them aside but will not completely reject them.
Les explains that the word "but" in the text points to the flip side: Israel will commit iniquity, yet God says His mercy will never depart from them as it did from Saul. Regardless of how far Israel falls into wickedness, God will discipline them. They will still face the greatest discipline any people have ever had when they go through the tribulation. But God is not destroying the nation of Israel, nor has He turned His back on them forever. He has set them aside. Even after the crucifixion, Peter addressed the children of Israel in Acts, pleading with them to accept that the one they crucified was their Messiah, but the nation rejected it. So God set them aside, yet not permanently.