Religion followed in the east was Zoroastrianism and west part of the byzantine empire was Christian.
Step-by-step explanation:
The byzantine empire was large and contained many ethnic and religious groups inside it who had their own religion which they were free to profess as far as they remained loyal to the empire they were being ruled by.
The eastern part of the empire was mostly Zoroastrian but they also had Buddhists and Hindus on the eastern fringes of the empire and they mostly had Christians in the western part of the empire which was taken over from Rome