Kartir was probably instrumental in promoting the cause of Mazdaism (as opposed to Zurvanism, the other - now extinct - branch of Zoroastrianism), for in his inscription at Naqsh-e Rajab, Kartir makes plain that he has "decided" that "there is a heaven and there is a hell", thus putting himself at odds with the principles of (fatalistic) Zurvanism. Nonetheless, it was during the reign of Shapur I (r. 241-272) - to whom Kartir was first appointed advisor - that Zurvanism appears to have developed as a cult, and this contradiction remains an issue of scholastic dispute.
Simultaneously, Kartir is also considered to have been a significant force in an iconoclastic movement that would result in the loss of favour of the shrine cults, an alien (to Indo-Iranian religious tradition) form of worship inherited from the Babylonians and instituted six centuries earlier by Artaxerxes II as an instrument for tax collection. It was during Kartir's time as high-priest that the shrines were - by law - stripped of their statues, and then either abandoned or converted into fire temples (see Atar).
According to his own inscriptions, Kartir rose to power during the reign of Shapur I (r. 241-272), to whom he served as advisor and accompanied on travels. Shapur's son Hormizd I (r. 272-273) appointed Kartir Moabadan-Moabad, 'priest of priests', a position Kartir ruthlessly used to promote his own position and to punish lower-ranking priests whose opinions he considered contrary to his own. Under subsequent kings, Kartir called for the persecution of adherents of other religions, in particular Manichaeans, whose prophet Mani was sentenced to death by Bahram I (r. 273–276), very likely on the instigation of Kartir and even though Shapur I had previously been a patron of the prophet. The persecution ceased during the reign of Narseh (r. 293–302), probably after the death of the high-priest.