Circumcision had value in the time of the old covenant (and, being the sign of Abraham, not of Moses, it has value still because the Abrahamic covenant has not been superseded). Its value is in being the outward sign of an inward reality. It judges those before God who have only the outward sign, and it commends those to God who have both inward and outward, the sign and the signified. And, as an outward sign, it physically differentiated the Hebrews from the other nations. They were set apart and alone had the Word of God revealed to them in the Law and the Prophets, as distinct from the nations. God gave them charge of the Law and charged them to teach it and apply it. Their faithfulness in this task (or lack thereof) did not alter the faithfulness of God. If every man were a liar, it would not alter God's truth, His trueness. Our faithlessness by contrast makes much of God's faithfulness and truth. Our unrighteousness makes much of His righteousness.
But if our unrighteousness serves to show the righteousness of God, what shall we say? That God is unjust to inflict wrath upon us? By no means! For how then could God judge the world and stem the tide of evil described in 1:18-25? But if, through my lie, glory is given and abounds toward God because of the greatness of His truth, why am I being condemned; am I not responsible for increasing His glory? By no means! Sin is still sin! How can evil take credit for the glory of God? The fact that God brings good out of evil neither justifies the evil nor absolves the evil-doer. He is glorified in spite of sin, not because of sin. God triumphs over evil by the death of Christ. He had to die to save us from evil and redeem us from sin.
As stewards then of the Word of God, are the Jews better off than the Gentiles? No, not at all, for it has already been established that all men, Jew and Greek, are under sin (2:12). To further this point, Paul quotes Ps 14, which groups Jew and Greek under the one banner:
"None is righteous, no, not one. . . . All have turned aside; together they have become worthless." All have sinned, Jew and Gentile, and every mouth is stopped from contending with the condemnation of God, because all are without excuse and accountable to God. Therefore, no one, no human being, Jew or Gentile, will be justified by works of the law because the law can only bring a clarifying knowledge of the magnitude of our own sin. No one seeks God, no one does good, so how could anyone ever become righteous by works of the law?
Our unrighteousness has served to show the righteousness of God, and the law, in its principles, is the codifying of that righteousness, the reflection of who God is and of His moral excellence. But now the righteousness of God has been made manifest apart from the law (embodied in the person of Jesus Christ, the incarnate Son), a righteousness that does not originate in us, but that comes from Christ and by faith in Him, who procured it by His perfect life, death, and resurrection; the righteousness of God, through faith in Jesus Christ, given unto all the believing ones, Jew and Gentile. There is no distinction, all have sinned and fall short of the glory of God; therefore, also, all are justified by His grace as a gift, through the redemption that is in Christ Jesus, whom God put forth as that which would avert the wrath of God from us in order that His wrath would be satisfied by Christ's blood rather than ours: a propitiation to be received by believing, by faith. God's righteousness then has been shown in absolving sinners and declaring them righteous because Christ paid for their sins with His blood and His righteousness is imputed to them by faith.
God can be both just and the justifier of the ungodly because sin has not been overlooked, nor has its penalty been ignored, but has been dealt with in the crushing of the Son. So God is just and holy in regard to the sin that has been removed from the account of the sinner. And God can also be the justifier of the unrighteous because a real exchange has been made wherein the sin has been removed from the sinner's account and paid for by the shed blood of Christ, and the real perfect righteousness of Christ has been imputed to him, so that God's attitude toward the justified is one of love, forgiveness, and delight, because He sees the righteousness of Christ which covers the sinner like a rich robe. He can be the justifier while also not ceasing to be just, according to His own holy standard.
Having been declared righteous by God who put our sin on Christ, having been given the righteousness of Christ and having had our sin atoned for, all boasting is gone. If one is justified by faith apart from works of the law, then all are (Jew and Greek), for all stand equally deserving of the wrath of God, and all stand equal recipients by faith of a perfect righteousness not their own, that comes to both from the same source: Christ, and by the same instrumental means: faith. The circumcised? Justified by faith. The uncircumcised? Justified through faith. Not by works of the law, so that no one may boast before God or one another. And the law then is not overthrown because the righteousness that comes by faith and satisfies the requirement for peace with God is a righteousness according to the law, kept by the perfect law-keeper, the Law-Giver.