In Romans 4 Paul illustrates his statement claim that God will justify all those who are in Christ Jesus by faith, no works. Like Galatians, he uses the well-known story of Abraham in order to show that the father of the Jewish people was himself made right with God without submitting to ritual (like circumcision) or keeping the Law.
Abraham was a prototype of righteousness in Second Temple Judaism. He perfectly kept the Law according to Sirach, a wisdom text written about 200 B.C.
Sirach 44:19–21 (NRSV) Abraham was the great father of a multitude of nations, and no one has been found like him in glory. 20 He kept the law of the Most High, and entered into a covenant with him; he certified the covenant in his flesh, and when he was tested he proved faithful. 21 Therefore the Lord assured him with an oath that the nations would be blessed through his offspring; that he would make him as numerous as the dust of the earth, and exalt his offspring like the stars, and give them an inheritance from sea to sea and from the Euphrates to the ends of the earth.
Jubilees 23:10 For Abraham was perfect in all of his actions with the LORD and was pleasing through righteousness all of the days of his life. (OTP 2:100)
Another reason for using Abraham in both Galatians and Romans is Paul’s opponents may have used Abraham as an example for the Gentiles. Abraham was a Gentile who was righteous before God. Why did God declare him righteous? They might answer because he obeyed God by not withholding his only son (Genesis 22) and because he submitted to the sign of the covenant, circumcision.
Paul’s main point in Romans 4 is simple. In Genesis 15:6 God declared Abraham righteous, before he was given the sign of the covenant (Gen 17) and long before the Law was given. For Paul, Gentiles are declared righteous just as Abraham was, by faith.
Are there other factors which may account for why Paul used Abraham as an analogy in Romans 4?