The theme of this epistle is that the righteousness of God is revealed in the Gospel. This is witnessed by and promised in the OT. In the OT the righteousness of God punishes sinners and provides salvation for those trusting in him.
Even though Gentiles know God from his revelation in nature, they refuse to acknowledge him. The Jews have God's law, but they do not keep it because they do not honor him. Therefore, the righteousness of God is revealed as his wrath in judgment (1:18-3:20).
On the other hand, the righteousness of God provides salvation for those trusting in him. God's actions are consistent with his character. He puts forth Christ Jesus as the propitiation for sin. Through faith believers become united with Christ. Their sins are forgiven, and they have the good standing of right relationship with God. This way of salvation does not contradict the law. Rather, it is in harmony with the law, correctly understood in the context of covenant (3:21-31). This is demonstrated in the life of Abraham. He was justified by faith and received circumcision as a seal of the justification he received while uncircumcised (4:1-25).
Being justified, we now have a good relationship with God and a firm standing in his grace. We can rejoice in our hope of sharing in God's glory in the future. Such hope is grounded in God's love for us and will not disappoint us. Therefore, we can rejoice even in our present sufferings (5:1-11).
This salvation is universally applicable. Just as Adam's sin affected all of his descendants, so salvation benefits all who are united with Christ by faith. Indeed, grace is much more powerful than sin (5:12-21).
If grace is much more powerful than sin, shall we continue to sin so that grace may abound? No, not at all! In order to experience grace, we have to be united with Christ in his death and resurrection. Christ died to sin and lives to God. So we need to live out this union by being dead to sin and alive to God (6:1-14). Furthermore, we are enslaved either to sin or to God. If we commit sin, we become enslaved to sin, which leads to death. But if we offer ourselves to God and become slaves of righteousness, we will reap holiness now and, in the end, eternal life (6:15-23).
When we died with Christ, we died to the law and were married to Christ in order to bear fruit to God. When we were living in the flesh, that is, on our own resources, we ended up in death. For in such a condition, sin takes control over us. Sin uses the law to arouse our passions and leads us to sin. The result is that we cannot do the good we want to do, but instead we do the evil that we do not want to do (7:1-25).
Because of our flesh (human nature under the control of sin), the law is not strong enough to deliver us from this misery. God sent his own Son to take on human nature and condemned sin in the human nature so that the righteous requirements of the law can be fulfilled in us who do not walk by the flesh but by the Spirit. Since we are led by the Holy Spirit, we are children of God and coheirs with Christ. If we share in his suffering now, we will share in his glory in the future. Our future glory is such that our present suffering is not worth comparing with it (8:1-27).
This scheme of salvation is not an afterthought. God planned it before the foundation of the world, based on his love for us. Therefore we can be confident that we will experience full salvation, including our future glory (8:31-39).
Faithfulness promotes right relationship. The righteousness of God includes his faithfulness. At present, the Jews are not saved. Is God, then, faithful to his promise to Abraham? Yes, indeed. We need to recognize that not all the descendants of Abraham are his heirs. God is sovereign, and he is free to determine the condition of heirship. That condition is faith. At present the unsaved Jews are unsaved because of their unbelief, not because of God's unfaithfulness to his promise. They insist on their own way to earn righteousness before God and reject the way of righteousness established by God (9:1-10:21).
God, however, has not completely rejected Israel. At present, some Jews are saved. In addition, God can use the Jews' present rebellion to bring about good for all. This occasions the proclamation of the Gospel to the Gentiles, which, in turn, makes the Jews envious. After the Gentiles are saved, the Jews as a whole also will be saved (11:1-36).
God is morally pure. Therefore, those who are in good relationship with God should be morally pure too. The power of the Gospel makes believers morally holy, and this is manifested in discerning and doing the will of God in daily life (12:1-15:13).