“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.”
Volume 1, Chapter 2 "Of the Union and Internal Prosperity of the Roman Empire, in the Age of the Antonines" http://www.ccel.org/ccel/gibbon/decline/files/volume1/chap2.htm. The portion regarding the views of the religions of the time taken by various constituencies has been misreported as Gibbon's own assessment of religion generally. See Paul F. Boller, John George, They Never Said It: A Book of Fake Quotes, Misquotes, and Misleading Attributions (1990), pp. 34–35.
The bold text has been misattributed to Lucretius and Seneca the Younger.
The Decline And Fall Of The Roman Empire (1776)
Source: The Decline and Fall of the Roman Empire
Context: 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.
Context: 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 embittered by any mixture of theological rancour; 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.
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Edward Gibbon 43
English historian and Member of Parliament 1737–1794Related quotes

Source: Dictionary of Burning Words of Brilliant Writers (1895), P. 344.

As quoted in What Great Men Think of Religion (1972 [1945]) by Ira D. Cardiff, p. 245. Actually said by Edward Gibbonː "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." (The History of the Decline and Fall of the Roman Empire, 1776, Vol. I, Ch. II).
Misattributed
“All religions were equal. But Islam was more equal.”
Freedom of expression - Secular Theocracy Versus Liberal Democracy (1998)
Context: The Emergency which Islam had imposed after its advent in India and which had caused resentment among Hindus for a long time, now stood fully sanctioned by the Hindu elite. All religions were equal. But Islam was more equal.

Speech before the New England Society (22 December 1843)
Possibly related to :
The Americans equally detest the pageantry of a king and the supercilious hypocrisy of a bishop.
Junius, Letter xxxv (19 December 1769)
It established a religion without a prelate, a government without a king.
George Bancroft on Calvinism, in History of the United States (1834), Vol. III, Ch. vi.
Oh, we are weary pilgrims; to this wilderness we bring
A Church without a bishop, a State without a King
Anonymous poem "The Puritans' Mistake", published by Oliver Ditson (1844).

This they said, and this meant. They did not mean to assert the obvious untruth, that all were then actually enjoying that equality, nor yet, that they were about to confer it immediately upon them. In fact they had no power to confer such a boon. They meant simply to declare the right, so that the enforcement of it might follow as fast as circumstances should permit. They meant to set up a standard maxim for free society, which should be familiar to all, and revered by all; constantly looked to, constantly labored for, and even though never perfectly attained, constantly approximated, and thereby constantly spreading and deepening its influence, and augmenting the happiness and value of life to all people of all colors everywhere. The assertion that "all men are created equal" was of no practical use in effecting our separation from Great Britain; and it was placed in the Declaration, nor for that, but for future use. Its authors meant it to be, thank God, it is now proving itself, a stumbling block to those who in after times might seek to turn a free people back into the hateful paths of despotism. They knew the proneness of prosperity to breed tyrants, and they meant when such should re-appear in this fair land and commence their vocation they should find left for them at least one hard nut to crack. I have now briefly expressed my view of the meaning and objects of that part of the Declaration of Independence which declares that "all men are created equal".
1850s, Speech on the Dred Scott Decision (1857)

The Master-Word In Medicine (1903)
Context: Though a little one, the master-word looms large in meaning. It is the open sesame to every portal, the great equalizer in the world, the true philosopher's stone, which transmutes all the base metal of humanity into gold. The stupid man among you it will make bright, the bright man brilliant, and the, brilliant student steady. With the magic word in your heart all things are possible, and without it all study is vanity and vexation. The miracles of life are with it; the blind see by touch, the deaf hear with eyes, the dumb speak with fingers. To the youth it brings hope, to the middle-aged confidence, to the aged repose. True balm of hurt minds, in its presence the heart of the sorrowful is lightened and consoled. It is directly responsible for all advances in medicine during the past twenty-five centuries. Laying hold upon it Hippocrates made observation and science the warp and woof of our art. Galen so read its meaning that fifteen centuries stopped thinking, and slept until awakened by the De Fabrica, of Vesalius, which is the very incarnation of the master-word. With its inspiration Harvey gave an impulse to a larger circulation than he wot of, an impulse which we feel to-day. Hunter sounded all its heights and depths, and stands out in our history as one of the great exemplars of its virtues With it Virchow smote the rock, and the waters of progress gushed out while in the hands of Pasteur it proved a very talisman to open to us a new heaven in medicine and a new earth in surgery. Not only has it been the touchstone of progress, but it is the measure of success in every-day life. Not a man before you but is beholden to it for his position here, while he who addresses you has that honor directly in consequence of having had it graven on his heart when he was as you are to-day. And the master-word is Work, a little one, as I have said, but fraught with momentous sequences if you can but write it on the tablets of your hearts and bind it upon your foreheads. But there is a serious difficulty in getting you to understand the paramount importance of the work-habit as part of your organization. You are not far from the Tom Sawyer stage with its philosophy "that work consists of whatever a body is obliged to do, and that play consists of whatever a body is not obliged to do."
A great many hard things may be said of the work-habit. For most of us it means a hard battle; the few take to it naturally; the many prefer idleness and never learn to love labor.

1800s, First Inaugural Address (1801)
Context: All, too, will bear in mind this sacred principle, that though the will of the majority is in all cases to prevail, that will to be rightful must be reasonable; that the minority possess their equal rights, which equal law must protect, and to violate would be oppression. Let us, then, fellow-citizens, unite with one heart and one mind. Let us restore to social intercourse that harmony and affection without which liberty and even life itself are but dreary things.

“True marriage is the union that mates
Equal with equal.”
Source: Prometheus Bound, line 890 (tr. G. M. Cookson)