Bardy, G. (Trans. P.W. Singleton). "Chapter Seven: The Expansion of Christianity." The Church at the End of the First Century. London: Sands & Co. 1938, 139-159.
Bardy confesses that we have little reliable information about the spread of Christianity in the late first century. He expects a good deal was moved along by the natural travels and resettlement of Christians who would tell about their Christian faith. During the apostolic period, many cities in Palestine and Syria gained a Christian population (Bardy 1938, 140). From the work of Paul, much of Asian Minor had communities which included Christians (Bardy 1938, 141).
By the time of Ignatius, many communities surrounding those mentioned in Acts or in the Epistles had a Christian presence. We know this not only from Ignatius but also from Pliny's letter to Trajan (Bardy 1938, 142). Polycarp's communication with Philippi suggests a continuing presence of Christianity in that area (Bardy 1938, 143).
We know relatively little about thestart of Christianity in Egypt, but Bardy finds accounts tying it to either Mark or Apollos (Bardy 1938, 144). The gnostic views of Basilides have a basis in Christianity, and there is some possibility that a number of early Christian writings come from Egypt. The region of Edessa also had a strong Christian presence at a fairly early time (Bardy 1938, 145).
In comparison to the East, Bardy finds Western Christianity to have spread more slowly, but that there were likely early movements into Gaul and Spain (Bardy 1938, 146).
By the end of the second century, Christianity was widespread in northern Africa. Bardy notes that we know nothing before 190, but that the spread must have started earlier (Bardy 1938, 147).
Bardy observes that the social expansion of Christianity may have been more important than the geographical expansion (Bardy 1938, 149). 1 Corinthians chapter one speaks to the social and cultural diversity of the Christians in Coirinth. Rich and poor alike were participants, as were slaves and free. The social spread, and the geographical spread, was not apparently carried on primarily by professional missionaries, but by a cross-section of the Christian adherents, who moved from place to place, bringing the Christian Gospel (Bardy 1938, 151). The movement seems to have all the marks of a grass roots organization, but with the order brought by the bishops, it proceeded in a measured manner (Bardy 1938, 152). The Christian movement was centered on the teaching delivered by the apostles, which brought considerable unity.