The Jews looked to Abraham as their hero and the Father of the Faithful. So Paul used Abraham as an example of righteousness (Romans 4). Paul went back to Genesis 15 where God instituted His covenant with Abraham. The Bible tells us that Abraham believed in the Lord and the Promised Messiah, and God accounted it to Abraham for righteousness. It is the same today. Abraham couldn’t be justified by his works, and so he had nothing to boast about. So we have nothing to boast about when we are only justified by Grace, through believing God’s Word, and trusting in Jesus Christ, who came in the flesh, and fulfilled all the promises, and prophecies of the Old Testament. Now God uses Christians like you and I to testify to others about how we learned to lean on God’s righteousness and power by His Grace.