Since Romans is mainly a Christian letter, written by Christians for Christians, most of the squabbles and infighting about its meaning come from Christians. Go figure.
Roman—it's right there in the name. You would think Catholics love Romans and, by extension, Paul. But you'd be wrong. It's not that Catholics don't dig Paul. But as far as the original followers of Christ go, Peter is much higher up on their list of apostolic crushes.
Peter gets to be the first Pope. He gets the keys to Heaven. He's the rock that built the church. Paul just gets to stand around holding a sword in his hand waiting to be beheaded. He even has to share his feast day—June 29th—with Peter. Talk about getting muscled out (source, 6).
But Paul is still the most prolific author in the Bible, so he hasn't been totally left out of Catholic thought and theology. His writings have had influence on lots of important Catholic thinkers through the years.
Augustine, for example, was especially fond of Paul. Not only did he convert to Christianity after reading a single line from Paul's letter to the Romans (it must have been a really good one), he also based some of his doctrine of original sin on Paul's thoughts about Adam eating that apple in Romans 5:12-21 (source).
Thomas Aquinas was also influenced by Paul, whom he called "the Apostle" (because they were so tight… no, not really). Aquinas believed that Paul's thoughts in Romans 2:15, pointed to a natural law that was written on the heart of every single person. This idea is still part of Catholic theology to this day. Go Paul.
Looks like it's not all snubbing and sword-holding from the Catholic Church when it comes to Paul, after all.
Protestants on the other hand—and we're including a wide net of non-Catholic faiths in this section—are really, really into Paul. In fact, you might say that Paul and his letter to the Romans started the whole Reformation. Gee, we wonder why Catholics don't like him as much.
Martin Luther pretty much got the ball rolling. Though he began as an Augustinian monk and a nice Catholic boy, Luther grew disillusioned with the feeling that he could never escape guilt and sin try as he might. But when he read Paul's letter to the Romans, he realized that he was just trying too hard.
Luther clung to Paul's idea of "justification by grace through faith" (3:28). It's the idea that "works"—in other words, doing stuff we think is holy to impress God—doesn't get us anywhere. Finally, Luther realized that all his Catholic piety and prayers couldn't bring him closer to God. He just had to open his heart to God's love and grace in order to be saved (source, 7-8). It was a nice thought.
The Catholic Church was less tickled. Since they were the only show in town, they weren't really open to criticism, to say the least. Luther persisted, though, and was eventually excommunicated. Thus, Protestantism (named because its adherents "protested" the Catholic Church) was born.
Other Christians began to follow in Luther's footsteps. John Wesley http://wesley.nnu.edu/john-wesley was so inspired by listening to Luther's commentary on Romans, that he decided to branch out and do his own thing, too. John Calvin http://www.iep.utm.edu/calvin also was pretty darn into Paul. These guys became the fathers of modern Protestant denominations such as Presbyterian, Baptist, United Church of Christ, and Methodist (source, 4). That's quite a reach Paul has.
Paul's been hugely influential to all different kinds of Christians. Though, actually, it is sort of ironic (and sad) that Paul, who was striving for Christian unity throughout Romans, would unintentionally create the biggest schism in the Christian church over 1,400 years later. Oh, irony. You're so nutty.
Paul's letters, especially his letter to the Romans, have led to loads of problems between Christians and Jews throughout the years.
Many not-so-nice folks have pointed to passages in Romans where Paul criticizes non-Christian Jews, talks up hypocrisy among the Jews, and details all the faults of Jewish law. To them, these verses confirm what they already suspect. Paul—and by extension God—thinks it's fine to hate and oppress Jewish people. Sigh.
Of course, this argument ignores the huge fact that Paul was Jewish. He states his proud Jewishness: "I myself am an Israelite, a descendant of Abraham, a member of the tribe of Benjamin" (11:1). He cites Hebrew scripture left and right. He honors the spirit of Jewish law. And, don't forget, he worships Jesus—the guy he saw as the Jewish messiah. So yeah, he's pretty down with Judaism.
Paul himself struggled his whole life with the idea that his Jewish brothers and sisters were not all rushing to accept Jesus as the Messiah. But in Romans especially, he refuses to denounce or reject them as a whole. Just take a look:
• "Then what advantage has the Jew? Or what is the value of circumcision? Much, in every way. For in the first place the Jews were entrusted with the oracles of God." (3:1-2)
• "Do we then overthrow the law by this faith? By no means! On the contrary, we uphold the law." (3:31)
• "There is no distinction between Jew and Greek; the same Lord is Lord of all and is generous to all who call on him." (10:12)
• "I ask, then, has God rejected his people? By no means![…] God has not rejected his people whom he foreknew." (11:1-2)
• "All Israel will be saved[…] As regards the gospel they are enemies of God for your sake; but as regards election they are beloved, for the sake of their ancestors; for the gifts and the calling of God are irrevocable." (11:26-29)
Given this background, we're guessing that Paul would be pretty horrified at the way his writings have been used over the years. One of Paul's biggest fans, Martin Luther, even wrote an essay called On the Jews and Their Lies. Sadly, he quotes from Romans to make some of his awful points.
Interpretation of Paul: You're doing it wrong.
Most Christians probably don't know that Islam really respects Christianity. Christians are called "People of the Book" along with Jews. The Qur'an actually mentions Jesus—he's called 'Iesa in Arabic—around 25 times. Nice name dropping, guys.
But as far as Paul goes, Muslims would probably part ways with him on a few key issues. While Paul thinks Jesus's death and resurrection are essential to understanding who he is, Muslims don't believe these events ever happened at all.
Even though most scholars think the crucifixion is pretty much a historical certainty, the Qur'an says that Jesus did not die at the hands of the Romans. They say that the enemies of Jesus would boast, "'Indeed, we have killed the Messiah, Jesus, the son of Mary, the messenger of Allah.' And they did not kill him, nor did they crucify him […] Rather, Allah raised him to Himself" (4:157-8).
Of course, Muslims don't mean any disrespect by this. It's actually the opposite. They think Jesus is far too great a guy to have been executed. Islamic tradition teaches that Allah just made it look like Jesus died on the cross, even though he actually ascended into heaven unharmed. It's inconsistent with Islamic ideas about God that someone so faithful would be allowed to suffer and die. Of course, Paul doesn't see it that way at all.
Aside from what it says in the Qur'an, some Muslims even have an antagonistic relationship to Paul. They believe that Jesus's true teachings (which, naturally, point to Islam) have been warped and distorted over the years. Who is the prime culprit for this? Paul. After all, he never even met Jesus and yet his writings and ideas about the guy have formed the basis for Christianity (source).
The Qur'an does promise that "those who believed and those who were Jews or Christians or Sabeans [before Prophet Muhammad…] will have their reward with their Lord" (2:62). But if people think you distorted God's true message, then we're not sure you're included. Sorry, Paul.