The influence of European Christians placed Near Eastern Christians in peril during the Crusades.From the mid-13th century, Mongol rule was a relief to Persian Christians until the Mongols adopted Islam.
The Arab Islamic conquest of Persia, in the 7th century, was originally beneficial to Christians as they were a protected minority under Islam.
However, from about the 10th century religious tension led to persecution once more.
Khosrau I married a Christian wife, and his son Nushizad was also a Christian.
When the king was taken ill at Edessa a report reached Persia that he was dead, and at once Nushizad seized the crown and made the kingdom Christian.
In 313, when Constantine I proclaimed Christianity to be a tolerated religion in the Roman Empire, the Sassanid rulers of Persia adopted a policy of persecution against Christians, including the double-tax of Shapur II in the 340s.
Christians were feared as a subversive and possibly disloyal minority.According to acts 2:9 in the Acts of the Apostles there were Persians, Parthians and Medes among the very first new Christian converts at Pentecost.Since then there has been a continuous presence of Christians in Iran.Through the Battle of Avarayr and the out coming treaty for example, the empire's large amount of Armenian subjects gained the official right to profess Eastern Christianity freely.Political pressure within Persia and cultural differences with western Christianity were mostly to blame for the Nestorian schism, in which the Church of the East was labelled heretical.From Persian ruled Assyria (Assuristan), missionary activity spread Eastern Rite Syriac Christianity throughout Assyria and Mesopotamia, and from there into Persia, Asia Minor, Syria, the Caucasus and Central Asia, establishing the Saint Thomas Christians of India and the Nestorian Stele and Daqin Pagoda in China.