Paul uses the personality of Abraham as an image of faith in contrast to the law and circumcision. Paul uses Abraham at the service of his soteriology by citing him as a Scriptural argument for justification by faith. At the same time Paul also uses Abraham to defend the inclusion of the Gentiles among the people of God (Ecclesiological) and for his own mission to the Gentiles (Ecclesiological). The key notes of Paul’s reference to Abraham are motif of ‘faith and promise.’
Promise of land so important in the Old Testament passages is left to side, while the promise of seed is focused not only on the nation of Israel but expanded to include the Gentiles. Although Abraham is considered the physical progenitor of the Hebrews (4: 1; 9: 7; 11: 1-2) this aspect is set aside by Paul in favour of a focus on Abraham as a spiritual progenitor of a spiritual race. Hence Abraham becomes father of all believers in Christ.
By showing that Abraham had been justified by faith. Paul made it clear that his teaching on justification was in agreement with religion of Old Testament, rightly understood as in Rom 3:31, and he set himself in opposition to the current view of late Judaism that God’s election of Abraham was a reward of for his keeping the precepts of God and for his loyalty at the time of his great testing, when he received the command to sacrifice Isaac.
Before the Mosaic Law existed, before Abraham himself had been circumcised, the childless patriarch was promised by God that his descendence would be as numerous as stars. He believed the promise and his faith were credited to him as just (Gal 15: 2-6). Abraham did nothing but believe. His circumcision received later, was simply designed which confirmed the fact that he had already being justified by his faith (Rom 4: 11). So, the covenant was made not because of the faith but because of faith.