Romans 4 explains that man is justified through grace by his faith. God's righteousness was given freely ( ie. like a gift) to Abraham simply because he believed and trusted in God, not because of any works or obedience to the law. "To the man who does no work , but trusts God who justifies the wicked, his faith is credited to him as righteousness"(v.5). Paul further explains by using a quotation from David, which speaks of the blessedness of the man to whom God credits righteousness apart from works, "Blessed are they whose transgressions are forgiven, whose sins are covered, Blessed is the man whose sin the Lord will never count against him" (v 6-8). Paul says again that righteousness promised by God "comes by faith so that it may be by grace and may be guaranteed ... to all those who are of the faith of Abraham"(v.16). "The words, 'it was credited to him', were written not for Abraham alone, but also for us, to whom God will credit righteousness--- for us who believe in him who raised Jesus our Lord from the dead... for our justification" (v.24-25).