Romans 4:13-25

The Promise, that Abraham would be the father of many nations is guaranteed by the resurrection of Jesus (vs 25). Paul continues explaining why faith, based on the gift of grace, guarantees that the same faith Abraham has is for everyone. The availability of inclusion into Abraham's "family" is open to Jew and Gentile simply by believing God's promise. That credits us as righteous, keeping in mind that God is the originator.