These various waves of immigration of Christian refugees help account for the fact that, although at the start of the seventh century Christians constituted nearly half the population of the Iranian Empire, about 75 per cent of these were Nestorians, 20 percent Miaphysite Jacobites and 5 per cent Melkites faithful to Chalcedon.
In retrospect, three factors contributed to the rapid success of the missionary efforts in the east: First, significant Jewish exile communities were living in Mesopotamia. Second, Aramaic served as the lingua franca from Antioch to Central Asia, enabling Aramaic-speaking missionaries to communicate easily with both Jews and non-Jewish merchants and members of the upper classes. Third, the close-meshed net formed by the various routes of the Silk Road eased the spread of the new religions. Although the Roman-Parthian border was generally tightly maintained, traders or missionaries disguised as traders could cross it unhindered. In fact, the missionizing of Mesopotamia, which moved from Edessa to Nisibis, Arbil, Seleucia-Ctesephon and Maishan, followed the land routes of the Silk Road. The sea routes then enabled the expansion of missionary activity to the islands of Kharg and Socotra, as well as – presumably – South India.Baumer, The Church of the East, p. 25
I was just sharing with a local believer the other day that it was only as recent as the 1500s that the number of Christians in Europe outnumbered the number of Christians outside it. In line with this, Baumer here shares a remarkable statistic, that while never having government control, Christians in the Zoroastrian Persian (Sassanian) empire at one point made up nearly half the population.
Also, take note that business as mission is not a novel concept at all, but a tried and true method of the early church as they sought to bring the gospel to the Middle East and Central Asia.