>>18139601
A good point of reference would probably be Japan. In various surveys, Japan is 70% Shinto, 70% Buddhist, and 70% non-religious at the same time, because religious affiliation as a concept is not native to Japan. It's often remarked that Japanese people are "Born Shinto, marry Christian, and die Buddhist" because various beliefs and rituals get merged together so often. Shinto itself is also a broad umbrella term for all traditional Japanese beliefs, but local shrines/temples aren't seen as competing with one another or out to make the other "not Shinto". This is also, from my understanding, similar to Hinduism and how Hinduism intersects with other beliefs like Buddhism in India (save for Islam, which both Hindus and Muslims see as mutually exclusive) or Chinese and Korean folk religions with Confucianism, Taoism, and Buddhism.

So there is likely a state-imposed religious view for the rest of Roman history until the empire's collapse. From there post-Roman states and the ERE continue to evolve with a shared religious heritage mixing with local traditions, their neighbors, and whatever locals come up with. The Eastern Roman Empire would be staunchly anti-Zoroastrian out of pure "We Hate Persia" attitudes, and if Buddhism spreads from the East into Iran then it might see its way into Anatolia and the Levant. Roman gods continue to be worshiped throughout the former Empire, but Jupiter in Britain likely ends up looking different than Jupiter in Syria. Various pagan traditions remain, sometimes on a super local level (i.e. this one town venerates this one god by doing this one thing, their neighbors down the river have no idea what they're talking about), and most people don't really see inherent contradictions in this. The idea of identifying as one and only one religion outside of being a priest is weird, and actively refuting the existence of all gods but your own is downright bizarre.