I take as a given that Paul is the author of the letter to the Romans, and indeed for the most part the answer to the question Who is Paul is found in his being the author of the letter to the Romans. Many, especially those on the evangelical side of the equation, see Romans as the most important book in the Bible. Some would argue that the predominant theme of Romans is the Righteousness of God. Some would argue that the predominant theme of Romans is the Justification of man. Given the common roots of those words in Greek man would argue both of the above. There are those who see Romans as Paul's Theological Last Will and Testament for the Church. Was Paul writing to the Roman Church or to the seat of power for the whole (known) world? I tend to harbour the view that Paul is writing to himself, that Romans is a pastoral epistle, and that this is Paul's internal dialogue between everything Paul was brought up to be, everything he understood himself to be, and everything he had laboured for and sought to achieve. His Ontology and his Teleology in Discourse. A dear friend of mine, a Panel Beater, answered to the question, What is the main theme of Romans with the reply "Just because the Jews are circumcised, doesn't mean that they are a cut above the rest." I would like to know what others think of Romans, the purpose and theme, etc.
The practical purpose of the letter to the Romans was to introduce Paul to the Roman church, so they knew his credentials as an Apostle, (he had not planted it and had never so far visited it), and to introduce his developed theology of salvation through Christ as a means of gaining their acceptance as a brother in the faith. Along the way he dealt with one or two practical problems of praxis and belief in Rome that had been reported to him. Some have deemed it a 'cathedral' of a letter. There is a Roman Road to Faith in the letter to the Romans. Rom.3:23 - "for all have sinned and fall short of the glory of God," Rom.6:23 - "For the wages of sin is death, but the free gift of God is eternal life in Christ Jesus our Lord." Rom.8:1 - "There is therefore now no condemnation for those who are in Christ Jesus." Rom.10:9-11 - "because, if you confess with your mouth that Jesus is Lord and believe in your heart that God raised him from the dead, you will be saved. For with the heart one believes and is justified, and with the mouth one confesses and is saved. For the Scripture says, “Everyone who believes in him will not be put to shame.” .
Romans is only tangenitally a letter -- it does have some personal and church business at the end. But mostly it is a long and technical theological treatise. We can speculate on why Paul wrote this epistle to the Roman church, which he did not found and to whom he was a stranger. I suspect that Paul always intended the epistle to the Romans to be a general (circular) letter, or at least that he fully expected the letter to be circulated in this way, and took advantage of the opportunity. As Rome was his stepping-off point for his (expected) missionary journey to Spain, he wanted his letter to precede him and prepare the ground, as it were. Romans is Paul's summa theologica, a systematic exploration of Christian theology, soteriology, and eschatology, contrasted with Jewish religious law and custom. I've long treated the book of Hebrews as a companion book to Romans, and consider it likely based on a sermon given by Paul (though Hebrews was probably written by someone else, the theology therein is purely Pauline) because Paul hammers home the point that the Law in the end did nothing for Jews because they could not follow it. The Law could only show the sin to them; it gave them no avenue to overcome the sin in themselves or reconcile them to God. Religious Jews ostensibly had a leg up on Gentiles (Rom. 3), but Paul shows that even Jews who had the Law could not follow the Law, so it availed them nothing. Only the grace of God, by faith, paid for by the sacrifice of Jesus Christ upon the cross, could save. Justification is main theme, but more importantly it is contrasted against the uselessness of works in terms of salvation. By faith alone in Christ alone we are saved. Sola fide, solus Christus. Paul in Romans also puts on a masterclass for Old Testament exegesis. If you want to know how to exegete the Old Testament, study the books of Romans and Hebrews (and the teachings of Jesus Christ himself in the Gospels, of course; Jesus taught from the OT constantly). The epistle to the Romans is magnificent -- this is Paul at the height of his rhetorical power, driven along by the Holy Spirit, laying down the foundation of Christian theology for all time. I'm not sure it's the "most important" book in the Bible, or even the NT; but it does contain the most systematic exploration of what Christian belief and practice are (or should be, anyway).
Whilst I see that, I am not so convinced it isn't Paul discussing this complex relationship with himself. Presumably, Romans was written around 57-58, Quite possibly after the third Gentile Mission Journey, whilst he was making plans for a 4th, to go even further, to Rome and onward to Iberia. As it happens the first destination was achieved involuntarily, and the second was not accomplished. He was arrested in Jerusalem, presumably in 58 and after some legal wrangling, he appealed to Rome, where he ended up around 60 AD and spent a couple of years under house arrest. The Great Fire of Rome was in 64 AD, and it seems he died after that, though probably before Nero married the slave boy in 67 AD. This was not well received in the Roman Community and may account for some of Paul's forthrightness in Chapter One on Same-sex relationships. I think chapter 7 really highlights something of the internal dialogue. I see Romans as less of a summa theological and more of a pastoral epistle, sharing something of his own struggles. I don't really think that there is a single answer to the question. I think it was Peter who suggested that Paul could be difficult to understand. In chapter 13 we see is discussion about our relationship with secular authority. Assuming he write that before incarceration, one wonders how he would have reflected on that in his time under arrest.
NIV Study Bible Notes, Fully Revised Edition [Biblegateway] Paul’s purposes for writing this letter were varied: (1) He wrote to prepare the way for his coming visit to Rome and his proposed mission to Spain (1:10–15; 15:22–29). (2) He wrote to provide a logically ordered presentation of salvation to a church that he had not personally founded. (3) He sought to address the tension between Gentiles and Jews in the Roman church by explaining the relationship between them in God’s overall plan of redemption.