Kartir began his career as an ordinary herbad, a low ranking priest, under Shapur I (r. 240 to 270), and by the reign of Bahram II, some four decades later, had accumulated titles that made him the effective head of the Zoroastrian priesthood across the Sasanian Empire. What is known of him comes almost entirely from his own words: four monumental inscriptions he had carved at Naqsh-e Rustam, Sar Mashhad, Naqsh-e Rajab and on the Kaba-ye Zartosht, each recounting his rise in his own voice and listing the offices he came to hold, among them mowbed of Ohrmazd and judge of the empire.
The inscriptions describe active religious enforcement rather than quiet devotion. Kartir records establishing fire temples across the empire and its conquered territories, strengthening the position of Zoroastrian priests, and suppressing rival religions and sects he names directly, including Jews, Buddhists, Hindus, Nazarenes, Christians, Baptizers and Manichaeans, whose founder Mani was executed during this period. Zoroastrian orthodoxy in the third century Sasanian Empire is, to a significant degree, Kartir's own account of what he did to make it so, which is both the inscriptions' value and their limit: no independent Zoroastrian voice from the period survives to check his version against.