Why does Paul write his letter to the Romans?
There are a number of reasons, not just a single one. One reason is that he wants to be assisted by them as he goes to Spain on a mission trip. But above all, he writes as the apostle to the Gentiles because of the grace that has been given to them, to write to establish them, and to make sure things are going well at the church in Rome, because that’s his responsibility. He had a unique view of his apostleship. Either he’s suffering from delusions of grandeur, or else he, in fact, has a unique calling and apostleship this way, much like Peter had as well.
So, this makes this letter then an unusually wonderful letter, perhaps the best all of Paul’s letters in this sense: here he writes to a church that he has not preached to, that he has not taught for the purpose of establishing them. So Romans is more like an epistle than a letter. It does not so much build on a prior relationship and on prior teachings, but he explains everything as carefully as he can to them. You don’t want to call it a systematic theology of Paul, because there are areas that he doesn’t deal with here. There’s not a lot of Christology, about the pre-existence of Jesus that you find in Colossians and Philippians, etc. But it is the most systematic and organized of Paul’s letters, one that doesn’t build on prior teachings that he has given them. He can build on traditions, and he talks about baptism – he builds on that. He assumes they have been taught many things. But he wants to make sure, and these are the things that are laid out in a more systematic form than any of his letters.
So, the occasion of this letter makes Romans far more of an organized, epistle-like letter than the occasional nature of the letters to the Philippians, Thessalonians, and Corinthians. To call it a systematic theology, however, would be too far. It doesn’t cover everything. But it is organized to help establish a church which has not received his theological teachings up to this time (at least not directly).