‘In light of these comments, it appears that Paul is interested in more than simply meeting with these Roman Christians. He evidently wants them to provide support — moral and financial — for his westward mission; possibly he would like to use Rome as the base of his operation to the regions beyond. But why would he need to provide such a lengthy exposition of his views in order to get their support? Don’t they already know who he is — the apostle to …the Gentiles? And wouldn’t they readily undertake to provide him with whatever assistance is needed?
This may be precisely the point. Either the Romans have only a dim knowledge of who Paul is or, even more likely, they have heard a great deal about him and what they have heard has made them suspicious. If this is the case, or at least if Paul believes that it is, then presumably their suspicions would relate to the issues that Paul addresses throughout the letter, issues such as whether Gentiles and Jews can really be thought of as equal before God and, if so, (a) whether God has forsaken his promises that the Jews would be his special people and (b) whether Paul’s “law-free gospel” to the Gentiles leads to lawless and immoral behavior (cf. Galatians).
The tone and style of this letter support the view that Paul wrote it in order to explain himself to a congregation whose assistance he was eager to receive. When reading through Romans carefully, one gets the sense that Paul is constantly having to defend himself and to justify his views by making careful and reasoned arguments (see, for example, 3:8; 6:1, 15; 7:1; etc.). Moreover, he makes this defense in a neatly crafted way, following a rhetorical style known in antiquity as the “diatribe.” This involved advancing an argument by stating a thesis, having an imaginary opponent raise possible objections to it, and then providing answers to these objections.’
source: Bart Ehrman, ‘Paul’s “Exceptional” Letter to the Romans (ehrmanblog). [excerpt] I reposted this part of his article because I found it most interesting. If Ehrman has a problem with this, he can contact me.