My comment is posted merely as an addendum to the good answers already given in the thread to the initial post.
Let a man grasp the design of the apostle for the sweep of his thought throughout the epistle, and certain difficulties (likely not all) will disappear.
Paul uses (by our numbering ch/v) 15vv of ch.1 to greet the Roman Christians and explain his aim. He sets out his gospel thesis in vv16-17. His first main point follows from 1:18, where he begins by writing about the revelation of wrath, which discussion continues through various branches to 3:20. All of this matter must be interpreted through the purpose statement of 1:18. This is the "bad news" portion of the letter, to be immediately followed by the "good news" euangelion or gospel purpose that comes in his second point.
That second point is begun in 3:21, with language that invokes the revelation of righteousness. For the next 8 1/2 chapters, through the end of ch.11, Paul distills his verbal proclamation of the gospel, his preaching of Christ, setting it out before the Roman Christians (where he never yet went to preach in person). More toward the close of the letter, we note the Apostle continues beyond the gospel proper to gospel-implications, i.e. to matters of Christian practice that flow out of the lives that have become united to the Lord Jesus and his perfect mediatorship.
The outline of Romans (excluding the greeting and epilogue) is essentially Guilt (1:18-3:20), Grace (3:21-11:36), and Gratitude (12:1-15:13). Indeed, if someone could keep the expressed will of God, then justification in God's sight should come through obedience; however, the whole first portion of the letter is written to indict every man without exception. That includes the obviously guilty and the ignorantly guilty; the indulgent Gentiles, moralists of any stripe, and even Jews in external covenant with God who have possession of the Law and ostensibly know its content.
Those categories cover the field of humanity, even by analogy; thus e.g., we can settle the modern, proud Christian who boasts in his external relation to Christ while defying his authority, nicely in the same final category as the Pharisees who boasted that they were not like other men. There is no way to escape the crushing indictment of God through the moral law, through the preaching of that law to all men. It reaches to the heart and motive of everyone, and finds us all guilty. In that condition, there is only one remedy: namely, a righteousness apart from the law. How will you come into possession of a perfect obedience? You cannot produce it, you cannot buy it, you cannot promise even a token for it.
Yet it is available, without money or price, as the pure gift of God. Those terms seem so good, so wonderful, even a bit "overgenerous;" and it is sad to see so many reject the gift in order to mock it, or offer some effort rather than take it, or insist on paying just a little something to reduce the debt (even just a little). In what way could the gift be improved by the recipient? Do you have no comprehension of the cost? Still, men will not have the one thing necessary on the Giver's terms. They must be given the very eyes to see--meaning the faith--unto apprehending both their guilt, and the grace that satisfies God's righteous requirement.
Only profound thankfulness pouring out of a humbled heart that now loves the Redeemer is able to render obedience that God accepts (even then!) on account of the saved's union with the Mediator.
