The policy of the emperors and the senate, as far as it concerned religion, was happily seconded by the reflections of the enlightened, and by the habits of the superstitious, part of their subjects. The various modes of worship, which prevailed in the Roman world, were all considered by the people, as equally true; by the philosopher, as equally false; and by the magistrate, as equally useful. And thus toleration produced not only mutual indulgence, but even religious concord.
The superstition of the people was not imbittered by any mixture of theological rancor; nor was it confined by the chains of any speculative system. The devout polytheist, though fondly attached to his national rites, admitted with implicit faith the different religions of the earth. Fear, gratitude, and curiosity, a dream or an omen, a singular disorder, or a distant journey, perpetually disposed him to multiply the articles of his belief, and to enlarge the list of his protectors. The thin texture of the Pagan mythology was interwoven with various but not discordant materials. As soon as it was allowed that sages and heroes, who had lived or who had died for the benefit of their country, were exalted to a state of power and immortality, it was universally confessed, that they deserved, if not the adoration, at least the reverence, of all mankind. The deities of a thousand groves and a thousand streams possessed, in peace, their local and respective influence; nor could the Romans who deprecated the wrath of the Tiber, deride the Egyptian who presented his offering to the beneficent genius of the Nile. The visible powers of nature, the planets, and the elements were the same throughout the universe. The invisible governors of the moral world were inevitably cast in a similar mould of fiction and allegory. Every virtue, and even vice, acquired its divine representative; every art and profession its patron, whose attributes, in the most distant ages and countries, were uniformly derived from the character of their peculiar votaries. A republic of gods of such opposite tempers and interests required, in every system, the moderating hand of a supreme magistrate, who, by the progress of knowledge and flattery, was gradually invested with the sublime perfections of an Eternal Parent, and an Omnipotent Monarch. Such was the mild spirit of antiquity, that the nations were less attentive to the difference, than to the resemblance, of their religious worship. The Greek, the Roman, and the Barbarian, as they met before their respective altars, easily persuaded themselves, that under various names, and with various ceremonies, they adored the same deities. The elegant mythology of Homer gave a beautiful, and almost a regular form, to the polytheism of the ancient world.
The philosophers of Greece deduced their morals from the nature of man, rather than from that of God. They meditated, however, on the Divine Nature, as a very curious and important speculation; and in the profound inquiry, they displayed the strength and weakness of the human understanding. Of the four most celebrated schools, the Stoics and the Platonists endeavored to reconcile the jarring interests of reason and piety. They have left us the most sublime proofs of the existence and perfections of the first cause; but, as it was impossible for them to conceive the creation of matter, the workman in the Stoic philosophy was not sufficiently distinguished from the work; whilst, on the contrary, the spiritual God of Plato and his disciples resembled an idea, rather than a substance. The opinions of the Academics and Epicureans were of a less religious cast; but whilst the modest science of the former induced them to doubt, the positive ignorance of the latter urged them to deny, the providence of a Supreme Ruler. The spirit of inquiry, prompted by emulation, and supported by freedom, had divided the public teachers of philosophy into a variety of contending sects; but the ingenious youth, who, from every part, resorted to Athens, and the other seats of learning in the Roman empire, were alike instructed in every school to reject and to despise the religion of the multitude. How, indeed, was it possible that a philosopher should accept, as divine truths, the idle tales of the poets, and the incoherent traditions of antiquity; or that he should adore, as gods, those imperfect beings whom he must have despised, as men? Against such unworthy adversaries, Cicero condescended to employ the arms of reason and eloquence; but the satire of Lucian was a much more adequate, as well as more efficacious, weapon. We may be well assured, that a writer, conversant with the world, would never have ventured to expose the gods of his country to public ridicule, had they not already been the objects of secret contempt among the polished and enlightened orders of society.
Edward Gibbon, “Chapter II: The Internal Prosperity In The Age Of The Antonines — Part I”, History of the Decline and Fall of the Roman Empire, 1782.