Christianity came to Rome before Paul, but we have very little idea of how it got there or how closely it was aligned with Jerusalem. As Luke tells the story, Christianity did more out from Jerusalem, to Samaria and Judea, then to major Diaspora Jewish communities – Antioch, then Asia Minor, Greece (Corinth) and finally Ephesus. Paul’s mission to the gentile world began at Antioch in the Synagogue and his normal strategy was to find the synagogue in a community in order to reach the Jews and God-fearing Gentiles first, then he moved into the marketplace in order to reach Gentiles.
It is possible that the Roman church was not Pauline in theology, having been founded by Jews after Pentecost. We know that the letter to the Romans was sent five years before this time to a mixed congregation of Jews and Gentiles, but we have no idea how that letter was received by the community in Rome.
Ben Witherington suggests Paul was the first to bring the gospel of grace through faith and gentile salvation apart from the Law to Rome (Witherington, Acts, 785). This is entirely possible, since the only reference we have to pre-Pauline Roman Christianity is Aquila and Priscilla (Acts 18) and the reference in Tacitus to Jewish rioting over Chrestus. It there appears as though pre-Acts 28 Christianity in Rome was quite Jewish.
The similar questions arise when thinking about the Jewish community. To what extent were the Jews in Rome in contact with Jerusalem? What authority did the Sanhedrin have over synagogues in Rome? (Or anywhere, for that matter. In Acts 9 the High Priest requests that Christians be turned over to Paul, he does not order the synagogue to do anything!) There is therefore a tension in Paul’s arrival – how will he be received? Have Jews from Jerusalem managed to arrive before him? If they had left about the same time as he did from Jerusalem they could hardly have traveled faster given the time of the year. Paul has no idea if he will meet Jewish Christians who are predisposed to attack him, or whether they will be like the Bereans, more open to his teaching.
This uncertainty does not seem to bother Paul. Once he finds lodgings in Rome he begins to meet with individuals in order to explain his presence in Rome and, likely as not, to explain his “side of the story.” He is still the apostle to the Gentiles and his imprisonment will permit him to reach the household of Caesar.