Unlike most of Paul’s letters, the occasion for the letter to the Romans is not obvious. Although there seems to be a clear purpose statement in 15:24-29, it is not clear why Paul would have written the bulk of the book to support that purpose. There is no indication he is responding to questions from the Roman church nor does he address reported problems in the church similar to 1 Corinthians or 1 Thessalonians. Paul has yet to visit the church and it does not appear he has had an influence on the church prior to this letter (unlike Colossians, for example, a church founded by a disciple of Paul).
The consensus view until modern scholarship is that the main reason Paul wrote Romans is to set forth his theology in clear terms. Paul begins with sin, then on to salvation by grace, the role of the law, sanctification and finally the practice of the Christian life. For many, Romans is as close to a systematic theology as we get from Paul. In fact, many modern systematic theologies follow this same general outline.
But if this is a “compendium of Pauline Theology,” there is a great deal missing (the resurrection of Jesus, for example), and it is difficult to account for Romans 9-11. Paul’s discussion of Israel is often treated like a digression from his main point, as if it could be dropped from the book without damaging Paul’s argument. One additional factor is fact Paul’s letters are all written in some historical and social context. He did not appear to write books for the sake of putting his thoughts down for future generations to read and ponder.
Is there a more specific reason Paul wrote Romans? Is Paul responding to a situation within the Roman Church? (I am heavily indebted to Colin Kruse, Romans, 8-9 for this section.) There are several suggestions for explaining Paul’s pastoral response.
First, since the Roman church was not established by an apostle, Paul wrote Romans to provide the church with an “apostolic presentation of the gospel (Fitzmyer, Romans, 75; Kruse, Romans, 8).” Paul would do this in person when he arrives in Rome, but the letter offers a “pre-read” for the church prior to Paul’s arrival.
Second, Christian Jews expelled from Rome by Claudius returned to find the house churches in Rome now organized much differently than the Jewish synagogue. The Jewish Christians found they were not the minority within a Gentile church. Paul therefore wrote Romans to encourage the Gentiles to live in harmony with Jewish Christian. But this suggestion has some difficult because there is no evidence Gentile converts had rejected distinctive Jewish practices. Unlike Galatia, it is possible the only Gentile converts in Rome were God Fearing Gentiles and quite happy with most Jewish practices.
Third, since the status of the Law is an important issue in Romans, Paul may have written because Christian Jews who continue to observe the law were now in conflict with law-free Gentile Christians. This is like other Pauline churches, but it is not clear Gentiles in the Roman church had rejected the Jewish Law. Nor is there evidence of Judaizers in Rome. Roman Gentile Christians do not seem to have struggled with Judaizers like the Galatia Christians did. Romans 14-15 is unclear on who the weak and strong are and vague about the actually issues at stake. There may have been some God-fearing Gentiles who kept some the Law and other Gentiles who came into the church who were not at all attracted to Jewish traditions.
Fourth, it is possible Paul did not consider the Roman Christians to have been “evangelized” yet. In Romans 15:15-16 Paul says he has written boldly to the church, so that “so that the offering of the Gentiles may be acceptable, sanctified by the Holy Spirit.” The letter therefore demands a response to the gospel from the Roman readers with respect to Paul’s understanding of the Gospel.
Fifth, it is also possible Paul wanted the Roman Christians to hear his gospel in order to draw them “apostolic orbit” (Kruse, Romans, 8). Since Paul is the apostle to the Gentiles, he may have felt the Gentile believers were part of his commission regardless of how they originally came to hear the Gospel. Perhaps Romans 15:20 is an apology for taking as long as he has to come to Rome, the largest and most important city in the Empire (Fitzmyer, Romans, 76). If the Roman churches had grown to the extent Nero could use them as a scapegoat (ten years after Romans was written), then Paul cold be accused of overlooking a significant population of Gentile Christians.
In summary, any of these suggestions (or a combination of them) could explain why Paul wrote the letter to the Roman church. But it is possible he was motivated to write the later because he was moving into a new stage of his apostolic ministry rather than to meet some pastoral need in the church. For many, Paul write Romans to promote his planned trip to Spain.