[wayne_shrugged]

Why were the Romans, so opposed to Christianity, so tolerant of Mithracism, which was from the arch-enemy Parthia?

Because early Christians were Jews, and the Romans had just destroyed the 2nd Temple in Jerusalem because of Jewish insurrection.

[DouglasKC]

Why were the Romans, so opposed to Christianity, so tolerant of Mithracism, which was from the arch-enemy Parthia?
Because early Christians were Jews, and the Romans had just destroyed the 2nd Temple in Jerusalem because of Jewish insurrection.

Exactly. Jewish animosity was rampant in the 1st century. Over time traditional Christianity abandoned anything that even looked "jewish"...including the holy days of the Lord Jesus Christ, the Lords's sabbath, the food laws and other biblical practices.