How do we know the doctrine of spiritual Israel is true?

In Romans 2:28-29, Paul clarifies that being a Jew is not merely an outward identity marked by physical circumcision, but rather an inward transformation of the heart that occurs through the Holy Spirit. This underscores the concept of spiritual Israel, wherein all believers in Christ—regardless of ethnic background—are considered the true children of Abraham. Galatians 3:29 reinforces this by stating that if we belong to Christ, we are Abraham's seed and heirs according to promise. This shift from physical lineage to spiritual lineage is vital for understanding God's redemptive plan as it reveals that membership in His covenant community is based on faith in Christ alone, not ethnic or cultural heritage.