Paul's Epistle to the Romans appropriately is the first book after Acts in our canon, for reason being that it is the philosophically-expressed definition of Christianity, that it was written to Gentiles in Rome, and that it is applicable to everyone in the world but reflecting the divinely-inspired foreknowledge that most Christians would likewise be Gentiles. The world was already going in that direction, through Paul's unlikely direction, and the true religion was destined to eventually become the ruling force officially of the world's ruling Roman Empire. God's guiding power above was sculpting His Kingdom on earth, to be delegated not to David's racial descendants but to his spiritual descendants, not to Abraham's corporeal children but to his children of the promise through Abraham's Seed, as part of the eternally great irony that was newly playing out. Generally speaking, the Jews and Gentiles had permanently switched places, with the Gentiles assuming a place in God's favor infinitely superior to anything the Jews had experienced in the Old Covenant times, and that for one reason and one reason only — Jesus Christ had implemented His New Covenant to be active then and forevermore.
As such, when Paul, decades after his personal conversion, at the tail end of his third missionary journey, composed his best and lengthiest epistle, he immediately followed his greeting to the Christian Gentiles (Rm. 1:1-17) with an account of what had been the dark days of Gentile past (Rm. 1:18-32). He was setting up the universally-severe nature of the problem in order to show how Christ had solved it for everyone and why He was expressing love for all by doing so. Paul did not go back to Adam just yet, which he would in what we now have chapterized as Romans 5, nor did he in this opening passage call up the pre-Jewish Abraham, whom he would assign to Gentiles in Romans 4. Instead he set the stage for analyzing the irony of overwhelming Gentile preeminence in Christianity by showing them where they had been ethically before Christ, when the Jews at least had the Law, but when both had no achievable way to the reconciliation with God that all men had needed since Eden.
Romans Chapter 1 Verses 18 to 20: "For the wrath of God is revealed from heaven against all ungodliness and unrighteousness of men, who hold the truth in unrighteousness; Because that which may be known of God is manifest in them; for God hath shewed it unto them. For the invisible things of him from the creation of the world are clearly seen, being understood by the things that are made, even his eternal power and Godhead; so that they are without excuse:" This reproach is upon all men but, reading between the lines, the passage brings in the Gentiles like the Gentile Romans and places them beside of the Jews as part of those "all men" whom God must cleanse of unrighteousness. He must make all human creation godly or else — blameless as is their Creator — because from the necessity of giving them free will, by which they received the ecstasy of self-aware existence, it also resulted that all men sufficiently misused their choice to unacceptably deviate from their godly origin in the immaculate divine Mind (suppl. Col 1:12-22). From this they all incurred the destructive wrath of God, although as we will see in Romans Chapter 4, the Gentiles received such wrath through death, whereas the Jews too received death but additionally were afflicted by sin imputed because of their having received the Mosaic Law. While the Gentiles did not have the Mosaic-quality commandments, they still should have known that there was a God who in His immeasurable power created life as good and of course demands that His creation follow suit by acting in propriety. This much has been manifest to all regardless of race since that beginning, inherently and unavoidably because they share the attribute of thought with the God who created them in His image.
That all men should have acted godly owing to their mental awareness of God the Father's nature, but shamefully chose to act more like mindless animals despite knowing better, is the point of the list of depravity that fills verses 21 to 32. That this first list like his greeting, and for the same reason as his greeting, is aimed more at Gentiles is evident by its inclusion of pagan idol worship, Greek-prevalent homosexuality, undisciplined straight sexual behavior, and many other vices besides that suggest a disregard for all ten of the commandments. In these actions, Paul felt it fair to say that "they knew God" but "they glorified him not as God," although the offer was not yet on the table for the Gentiles to meet God in even an Old-Covenant type of relationship, but even still that "they which commit such things are worthy of death" (Rm. 1:32). The Gentiles' behavior before the gospel was therefore worse than the Jews' behavior, theoretically if degrees of unrighteousness from God mattered, because they lacked the Law, but as it would turn out, this would prove most fortunate for the Gentiles when both Gentile and Jew were offered the New-Covenant relationship with God through Jesus Christ. His blood would conquer each and every one of these sins, of any quantity, and would conquer the penalty of death in exchange for a simple returned allegiance. The Gentiles would see it as more to gain, because they were less confused about their own lack of merit, because they knew that they needed a fresh start in God's way of life from their past debauchery, and most of all because the Gentiles did not have a Mosaic Law's unsalvatory way to mix up with Jesus who is the only Way to the Father and to true salvation.
