Before I get into the details of the question, let me set out the scenario. At some point in the 3rd Century, Persia converts to Christianity in a "Light shines East" style scenario, where Christianity is largely merged with Zoroastrianism and Ahura Mazda is conflated with the Christian God. The Yazata probably take on the general role that saints/angels do in our timeline. It is roughly in this same time period that the Sassanids go on to conquer the Roman Levant. However, I'm not sure whether or not Egypt or Anatolia would be conquered in such a scenario but I'm assuming that at least the former probably will? In such a scenario, how would the other religions of the near east and Mediterranean at this time be affected. Here are some assumptions I already have.
Mithraism would likely not do very well. Even in the case that the Yazata are not still worshiped in a Christian Persia, Mithras would still be likely associated with Persians that militarily humiliated the Roman Empire and conquered a large part of the latter's territory. As such, I think that this cult would likely have too much political baggage to be palatable to the average Roman.
On the other hand, I think that certain brands of "Gnostism", such as Marcionism, might end up getting a political boon out of this in the Roman Empire, should they play their cards right. Especially if they betray the god of the Christian-Zoroastrian Persian God as the malevolent Demiurge that is in opposition to the Monad.
However, I'm not sure that Gnosticism would be tolerated within the empire's territory. Especially strains that consider either the god of Isreal/the creator to be a malevolent deity(Marcionism or Sethianism) or ones that consider Christ to be a false prophet(Mandeanism).
I'm not sure how Egypt-based religions (like the Cult of Isis and Hermeticism) and Anatolian-based religions (like the Cult of Cybele of Attis) would fare, regardless whether or not their respective lands would be conquered by the Persians. On one hand, I'm not sure how the Christian Sassanids would treat such religions in their conquered territories. Then again, some of these religions had aleady apparently spread through the rest of the Roman Empire at that point.
Mithraism would likely not do very well. Even in the case that the Yazata are not still worshiped in a Christian Persia, Mithras would still be likely associated with Persians that militarily humiliated the Roman Empire and conquered a large part of the latter's territory. As such, I think that this cult would likely have too much political baggage to be palatable to the average Roman.
On the other hand, I think that certain brands of "Gnostism", such as Marcionism, might end up getting a political boon out of this in the Roman Empire, should they play their cards right. Especially if they betray the god of the Christian-Zoroastrian Persian God as the malevolent Demiurge that is in opposition to the Monad.
However, I'm not sure that Gnosticism would be tolerated within the empire's territory. Especially strains that consider either the god of Isreal/the creator to be a malevolent deity(Marcionism or Sethianism) or ones that consider Christ to be a false prophet(Mandeanism).
I'm not sure how Egypt-based religions (like the Cult of Isis and Hermeticism) and Anatolian-based religions (like the Cult of Cybele of Attis) would fare, regardless whether or not their respective lands would be conquered by the Persians. On one hand, I'm not sure how the Christian Sassanids would treat such religions in their conquered territories. Then again, some of these religions had aleady apparently spread through the rest of the Roman Empire at that point.