We tend to think of Iran as one of the large Islamic countries in the Middle East, but that perspecitve is relatively recent in history. The area has been known as Iran for only a few years, but for centuries it has been called Persia. For 600 years, its most popular religion was Christianity.
Persia is home to two large groups of Christians, the Assyrian Church of the East and the Syriac Orthodox Church. Although these churches were originally maintaining ties with the Christian churches in the Roman Empire, they were indeed quite different from the churches in the Roman Empire. One reason for this is language.
Another factor that the churches within the Persian Empire did not maintain such close ties with their counterparts in the Roman Empire, was also the continuous rivalry between these two great empires. And quite often, Christians in Persia were (often falsely) accused of sympathizing with the Romans, even though the Roman Empire was persecuting and killing Christians, while the Persian Empire embraced Christianity. In Persia, unlike Rome, it was both legal and popular for people to leave Zoroastriansim (the mythological belief system of Persia prior to Christianity).
But it was not until the Council of Ephesus in 431 A.D. that the vast majority of Christians in Persia broke their ties with the churches in the Roman Empire.
Most of the Christians in the Sassanid empire lived on the western edge of the empire, predominately in Mesopotamia, but there were also important communities on the island Tylos (present day Bahrain), the southern coast of the Persian Gulf, the area of the Arabian kingdom of Lakhm and the Persian part of Armenia. Some of these areas were the earliest to be Christianized; the kingdom of Armenia choose the Christian faith sometime before 100 A.D., and became the first independent Christian state in the world in 301 A.D.; while a number of Assyrian territories had almost become fully Christianized even earlier during the 1st century, they never became independent nations.
Most Christians in the Persian Empire belonged to a number of predominately Christian ethnic groups. Some of these groups were the Assyrians, the Arabs of southern Mesopotamia, the Armenians, as well as some smaller ethnic groups such as the Syriacs. The latter group was taken to Persia as prisoners of war from the many conflicts with the Roman Empire. Conversion was common among ethnic Persians and other ethnicities residing in the empire. Among them were certain small Caucasian and Kurdish tribes which had converted to Christianity.
For approximately 600 years, these groups lived as Christians. In 651 A.D., the first wave of "jihad" from the Islamic Caliphate swept across Persia in a fourteen-year-long bloodbath. Christians could flee, convert to Islam, become enslaved, or be killed. Eventually, the name "Iran" would replace "Persia" as the usual name for the region.