 Yazdegerd I, like all other Sasanian rulers, was an adherent of Zoroastrianism.One of his predecessors, the powerful Sasanian shah Shapur II (), was thought to have brutally persecuted the Christians of Iran from 340 to 379 in a "Great Persecution".Although later shahs – Yazdegerd I, Bahram V (), Yazdegerd II (), Peroz I (), Khosrow I () and Khosrow II () – were also said to have persecuted the Church of East, the church quickly expanded.