Roman soldiers adopted various deities they encountered while on those long deployments (which often lasted years); also, conquered peoples were free to move around the Empire, and took their deities with them. The Jewish population of the world was nearly as high in Roman times as it is now, and at its peak was perhaps 15 per cent of the entire Empire’s population. :’)
One of the best preserved stories (that is, able to be reconstructed from recovered remains) that comes down from the 79 AD eruption of Vesuvius is the flight of the personnel from the temple of Isis — a bag containing all or part of the temple’s treasury was found from the last of the staff to succumb. Others from the temple (not certain how they were identified) had been overcome by the gases while fleeing.
In a way, the free and safe travel that was possible in Europe and the Middle-East under the Romans has never been duplicated