For 1500 years, Israel had been under the heavy yoke of the law. This had been their rule of life since the time of their coming out of Egypt when God entered into covenant with them at Mt. Sinai. Paul was clear of one thing (himself being a Pharisee for many years), by the works of the law shall no man be justified in the sight of God! If any man was going to stand a chance of gaining right-standing with God, it was to be by faith alone that the reward might be of grace!
In this fourth chapter of Romans, Paul seeks to address this seismic shift in religious worldview by pointing out that such a concept of imputed righteousness on the grounds of faith is nothing new! Abraham, the father of the Jewish nation predated Moses by some nearly 1500 years, yet Abraham had this testimony before God, that he was counted righteous on the grounds of his faith and not by the works of law. Paul lays hold on this truth and expounds this great doctrine of justification by faith by demonstrating that righteousness must come by means of imputation alone and can never come by the works of the law!