In the third century, Christianity spread into the Persian Empire, where it became a powerful presence. The means by which this auspicious event occurred are startling and even humbling for anyone who thinks in terms of deliberately planned missionary efforts. At least at first, many, perhaps most, of the Christians who found themselves under Persian rule really had no wish whatever to be there.
Man proposes, and God disposes.
I described the major Christian growth on the Empire’s western borderlands, which divided the Persian and Roman realms. Very significant for religious history were the wars and conflicts that raged over these regions from the first century through the sixth, and which became still more fierce under the Sassanid empire (224-651 AD). War meant enslavement, deportation and population movement, which brought reluctant populations into new lands. Here, though, they introduced new faiths, often to the horror of their new masters. As I have described in the past, Christian slaves played a potent role in spreading the faith into Caucasian lands like Iberia and Georgia, part of what I call Daniel Syndrome. Later, borderlands adopted particular forms of faith to appeal to one superpower, and to seek help against its rival.
Once war broke out, it was usually fought in the same general areas, where Christianity was both very strong, and exceedingly diverse. In modern terms, this meant northern Iraq, eastern Syria, and especially eastern Turkey, with the holy city of Nisibis a pivotal prize. If you find a strictly current map of where the US and its allies are fighting ISIS, you get a pretty good idea of the landscape.
Ironically, the better the Persians did in war, the more territories and populations they gained in these disputed areas, and the greater the unwanted spread of Christianity into their own empire. If they gained a substantial Christian minority, they had only themselves to blame. It was not so much that Christianity came to the Persian Empire: the Empire came to them.
The main “culprit” for Christian expansion was king Shapur I, son of the founder of the Sassanid dynasty, Ardashir. Shapur’s wars against the Romans raged through the 240s and 250s, culminating in his spectacular capture of the Roman Emperor Valerian. Briefly, Shapur even occupied Antioch itself. He conquered and deported many peoples, bringing them home to Persian soil.
Not surprisingly, then, by the late third century, the Persian Empire found itself with abundant Christians, drawn from a wide variety of sources and ethnic traditions.
I’ll describe these populations in more detail shortly.
Useful readings on these matters include Jes Peter Asmussen, “Christians in Iran,” in Ehsan Yarshater , ed., The Cambridge History of Iran Volume 3: The Seleucid, Parthian and Sasanid Periods, Part 2 (Cambridge University Press , 1983), 924-48. Also Michael H. Dodgeon and Samuel N. C. Lieu, eds., The Roman Eastern Frontier and the Persian Wars AD 226-363: A Documentary History (Routledge, 2002); Geoffrey Greatrex and Samuel N. C. Lieu, The Roman Eastern Frontier and the Persian Wars AD 363-628 (Routledge, 2005); and Beate Dignas and Engelbert Winter, Rome and Persia in Late Antiquity: Neighbours and Rivals (Cambridge University Press, 2007). Christian Van Gorder has a valuable book on Christianity in Persia and the Status of Non-Muslims in Iran (Lexington 2010).