From “History of the Apostolic Church” by Afshin Khorrami
Under the Parthians, the Apostolic Church had gained a considerable following in Mesopotamia, and was making inroads in Persia. Under the new Sassanid Empire, founded by King Ardashir of Pars, Zoroastrianism enjoyed a revival as the new state religion. The primary affect of Ardashir’s Zoroastrian revival was the decline of polytheism within Persia, while Christians and Jews were largely left untouched. In the year 240 AD, Shapur I outdated succeed Ardashir as Shah of Persia.
Shapur, like his father before him, was a Zoroastrian, however, he would eventually come into contact with the Patriarch of Ctesiphon, Mar Mani[1] of Babylon. Mar Mani would become an important member of the Sassanid court. Mani would write the Shabuhragan, or Book for Shapur, explaining the already established Church doctrine that Ahura Mazda was simply the Christian God as known to the Persians, and how Cyrus had freed the Jews from captivity in Babylon and how the three Zoroastrian Magi cisited the baby Jesus. Therefore, Mani argued, Christianity was simply a continuation of Persia’s ancient history, and that conversion to Christianity would be the logical endpoint of the Persian revival that began under Ardashir.
Shapur would finally be convinced after seeing a vision of the cross after his victory against the Romans at the Battle of Rasaena[2]. Shapur would then be baptized by Mani by the waters of the Jordan River, giving himself the title “King of Kings by the Will of God.” Although Shapur himself wa now Christian, he declared that Christianity, Judaism, Mandaeism, and Zoroastrianism would all have co-official status under the Sassanids. In Shapur’s mind, all four of these religions could trace their origins to the same God, and thus all deserved to be recognized by the Persian state. However, heretics of all four religions were brutally persecuted. Now that Shapur had converted, the world would never be the same again.
[1]Looks like I’ve found a use for Mani ITTL
[2]IOTL, the battle wa a Roman victory