Both Abraham and David were Jews. They had the sign of the Abrahamic Covenant - they were circumcised, the chosen people, and in a place of privilege. God's grace is pure grace. God does not favor the Jew over the Gentiles when it comes to salvation. Justification has nothing to do with being a Jew.
Man is justified apart from the deeds of the law.
Once you have the law, there are consequences of breaking it, but it is not a factor in justification. Baptism is good, but it does not save a person. Baptism follows salvation to show a person's identification with Christ, that he dies and is resurrected in newness of life with Christ. B asis for salvation, M eans of salvation, O bject of faith, C ontent of faith. Content is what God reveals at a certain time in history.