On 3:27, Dunn writes:
Once the centrality of 'faith' in the preceding section is grasped, the movement of thought in v27 becomes clear, for in the resumed diatribe of vv27-31 it is precisely this point which is hammered home: faith as the proper understanding of the law, faith as the indispensible basis of 'doing the law'.
Justification has always been by faith. In the New Age, both Jews' and Gentiles' faith must be in Jesus (Paul's Christology is clear here). In fact, those Jews justified under the Law (their 'doing' of the Law a result of their circumcision of heart, or faith) were also redeemed by Jesus Christ because God had merely passed over their former sins in his forbearance. But now, atonement has been made, and redemption accomplished in Christ.
Romans 3 adequately demonstrates the obstacles in interpreting Paul and the Law. The interpreter is ranging across eschatological and covenantal boundaries. At once dealing with Jewish legalism and obedience under the Old Covenant, and debates on the Law, then with the New Covenant in Jesus Christ, and justification through faith in him.