But it must be remembered that these are exceptional cases of intolerance revealed by Roman history. The general policy of the empire, on the other hand, was of extreme tolerance and liberality. The keynote of this policy was that all religions would be tolerated that consented to live side by side and in peace with all other religions. There was but one restriction upon and limitation of this principle, that foreign religions would be tolerated only in their local seats, or, at most, among the races in which such religions were native. The fact that the worship of Serapis was left undisturbed on the banks of the Nile, did not mean that the same worship would be tolerated on the banks of the Tiber. An express authorization by Rome was necessary for this purpose. Said authorization made said worship a _religio licita_. And the peregrini, or foreigners in Rome, were thus permitted to erect their own altars, and to assemble for the purpose of worshiping their own gods which they had brought with them. The reverse side of this general principle of religious tolerance shows that Roman citizens were not only permitted but required to carry the Roman faith with them throughout the world. Upon them, the Roman state religion was absolutely binding; and for all the balance of the world it was the dominant cult. "The provinces," says Renan, "were entirely free to adhere to their own rights, on the sole condition of not interfering with those of others." "Such toleration or indifference, however," says Döllinger, "found its own limits at once whenever the doctrine taught had a practical bearing on society, interfered with the worship of the state gods, or confronted their worship with one of its own; as well as when a strange god and _cultus_ assumed a hostile attitude toward Roman gods, could be brought into no affinity or corporate relation with them, and would not bend to the supremacy of Jupiter Capitolinus."