Parthian religion might be best described with two words: inclusive and evolving. As Parthia's empire held within it a variety of cultures, the Parthians wisely left each to their own beliefs and traditions, like the Seleucid Empire and the Persian Achaemenid Empire before them. While Parthia allowed Jews and Christian enclaves to thrive, the three leading beliefs were the Greek pantheon, Zoroastrianism, and Mithraism. But what did the Parthians themselves believe? Considering they left no historical record as to their cultural heritage, discovering the Parthian belief system may seem challenging, but starting with what was practical and prominent in their daily lives and considering from where they came, to where they were going, will help. Initially, it appears, as Persian and Greek cooperation were essential to the success of the Parthian Empire, the Parthians flirted with keeping both nation's customs, but once they stood on their own, they began to identify with Mithra.
This is a companion discussion topic for the original content at https://www.worldhistory.org/Parthian_Religion/.