This is something I wrote in a Facebook comment. I would like to hold on to it, and I hope it helps you understand Romans and the whole New Testament.
There is a clear origin in St. Anselm for the idea that Jesus had to die in order for the Father to forgive sin. (In other words, the origin of that doctrine is not the apostles, but from a Roman Catholic who lived in the 11th century.) Before that, as a reader of the church fathers, I do not think that doctrine existed. How could it? God forgave sin all through the Old Testament, and he is continually described as a merciful God. King David even said that God did not want sacrifice but repentance! (Ps. 51:16-17). And again, “to obey is better than sacrifice” (1 Sam. 15:22).
Instead, Jesus died to free us from slavery to sin. God was already merciful. Look at his description of the judgment in Ezek. 18:20-30. Even in the Old Testament, a person who repented and did righteousness could be forgiven and receive life. The problem is that so few of us actually lived righteously. That is Paul’s point in Romans 3 and Romans 7. The problem is us, and the problem is so bad that even a good, perfect, and holy law could not fix us (Rom. 7; Gal. 3:21). But what the Law could not do, God did by sending his Son in the likeness of sinful flesh as an offering for sin (Rom. 8:3). The law could not make us righteous, but the sacrifice of Jesus can make us righteous. Thus, when the Gentiles first believed, the Jewish Christians in Jerusalem remarked, “So God has granted repentance leading to life to the Gentiles as well” (Acts 11:18).
The gift of the atonement, and even of grace, is the power to live righteously and thus receive the ongoing mercy of God, which was always available even under the Old Testament (Ex. 34:6-7). Jesus did not die to change God or cause God to be free to give mercy. Jesus died for us! We were the problem, not God. Look at the description of the atonement in Titus 2:11-14, which describes both grace and the purpose of Jesus’ death. The gift of grace is that sin no longer has power over us (Rom. 6:14).
That is why even after Jesus died for us, Paul says three times that the unrighteous will not inherit God’s kingdom (1 Cor. 6:9-11; Gal. 5:19-21; Eph. 5:5). Two of those times, he says not to be deceived about that (1 Cor. 6:9; Eph. 5:6), and the other time he tells the Galatians he has been saying this repeatedly (Gal. 5:21). John tells us not to be deceived that it is only those who are practicing righteousness who have the imputed righteousness of God (1 Jn. 3:7). Yet, we have been deceived about all these things that the apostles warned us not to be deceived about. Why? Because we think Jesus died to change God rather than dying to change us.