How do we know all Israel shall be saved is true?

The phrase 'all Israel shall be saved' in Romans 11:26 refers not just to the ethnic nation of Israel but to the spiritual Israel, which encompasses all of God’s elect, Jew and Gentile alike. This illustrates God’s faithfulness to His covenant people. As it is written, this salvation is guaranteed by God's sovereign decree, meaning that it is not contingent upon human action but upon God's purpose and calling (Romans 8:30). Thus, those chosen by God will inevitably be saved, fulfilling the promise that He shall take away their sins (Jeremiah 50:20).