“The well-taught philosophic mind
To all compassion gives;
Casts round the world an equal eye,
And feels for all that lives.”

On a Girdle; reported in Bartlett's Familiar Quotations, 10th ed. (1919).

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.

An Old Man's Thoughts on Many Things, Of Education I
Context: Some distinguished philosophers think that boys' eyes should be taught or trained to the examination of objects: in other words, that boys should be taught to observe things and to see likeness and difference. It is done to some extent by all boys: their games teach them something, and they know a cake from an apple. But the power of careful, patient looking at a thing is not fully acquired without some pains on the part of a teacher. When a boy reads aloud, he must look carefully at the words and letters, or he will blunder. This is an instance of observation. But the philosophers mean, I believe, that we should introduce certain things called sciences into school teaching.

The Middle Temple Gardens
The Vow of the Peacock (1835)

Source: Read Me Like a Book

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

As quoted in page 184 of book Thapa Politics in Nepal: With Special Reference to Bhim Sen Thapa, 1806–1839 https://books.google.com.np/books?id=7PP1yElRzIUC&dq=bhimsen+thapa&source=gbs_navlinks_s|

La Parure (The Necklace) (1884)

World-Destruction: Never, Impossible! (1994)