Ro 4:4-5. This principle is now further explained. Paul reasons that justification by works rests on the principle that men may earn their salvation by doing good. If this principle were true, good men would be saved by their good works and salvation would not be a gift at all. But justification by faith rests on the principle that God imputes righteousness to the ungodly as a free gift. Salvation is not, therefore, earned by the sinner but is freely given to him when he puts his faith in the blood of Jesus Christ.
Ro 4:6-8. Paul has made a case for Abraham's justification apart from works, now he strengthens that case with another Old Testament illustration. The purpose of introducing David's testimony is twofold: (1) The Jews' law regarding two witnesses (Deut 19:15; referred to by Jesus in Mt 18:16 and by Paul in II Cor 13:1 and I Tim 5:19). David corroborated what is said about Abraham and further illustrates salvation apart from works; (2) David gives witness that the same principle of justification was operative even for those living under the Mosaic law.