This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.
The earliest Christians in Rome were Jewish residents there with ties back to Jerusalem, and the later tradition that Peter himself founded the Roman church around 30 to 33 CE is exactly that, a tradition rather than a verified fact, and it is best named honestly as tradition rather than stated as settled history. By 57 CE Paul was writing to the Roman Christians as an already going concern he had not founded, addressing them directly in his own letter, and by 64 CE the community was prominent enough that the emperor Nero could scapegoat it for the Great Fire of Rome, an episode the Roman historian Tacitus records directly in his own account.
Despite recurring persecution under emperors including Decius and Diocletian, the Roman church kept growing through the following centuries into one of the empire's major sees, developing organised clergy and its own property well before Constantine's toleration of Christianity in 313 CE removed the legal danger entirely.
What this changed for Christianity's spread was the survival and steady growth, across nearly three centuries of intermittent state hostility, of the church that would become the Western tradition's own leading see. A community that began as Jewish migrants meeting quietly in a hostile capital outlasted the very persecutions meant to destroy it and emerged, by 313, as an established institution ready for imperial favor rather than imperial fire.