It is likely that the voluntary Christ associations in Rome were indebted to Jewish missionaries who spread the gospel to them. We already hear about a dispute among the Jews over someone named Chrestus that lead the emperor Claudius to expel a number of Jews from Rome in 49 CE (Seutonius, Claudius 25), which is arguably a misspelling of Christos or “Christ” and had the Jewish Christ followers Priscilla and Aquila among the victims of the expulsion (Acts 18:2). It is probable that the creed that Paul cites in Romans 1:3-4 about how Jesus was a descendant of king David and that his powerful rule as the royal Son of God was exemplified by his resurrection from the dead was a well-known belief that Paul held in common with these other Jewish believers.
Paul admits in his letter to the Romans that he did not have firsthand acquaintance with the Christ associations in Rome and that he would prefer to not build on another person’s foundation (see Rom 1:8-13; 15:18-32). He wrote the letter to introduce himself and his theological ideas to the congregations in Rome, to encourage unity between Jewish and non-Jewish Christ followers in Rome, and to raise funds for a future venture to Spain. It is possible that Peter was one of the missionaries who laid the foundation for the gospel in Rome, but Paul does not explicitly specify this. Moreover, Paul does not ask his recipients to send greetings to Cephas or Peter, which would be odd if Peter was already there by 57 CE. Perhaps this is an argument from silence or just an indication that Peter had not yet arrived in the capital.