“Defending the truth is not something one does out of a sense of duty or to allay guilt complexes, but is a reward in itself.”

Source: Temple of the Winds

Not found in Augustine's works, it is stated in Fauxtations: Because sometimes the Internet is wrong : St. Augustine: The Truth is Like a Lion (18 October 2015) https://fauxtations.wordpress.com/2015/10/18/st-augustine-the-truth-is-like-a-lion/, that this is very likely a summary derived from statements of Charles Haddon Spurgeon about the "Word of God" or "the pure gospel", and the Bible:
:: The Word of God can take care of itself, and will do so if we preach it, and cease defending it. See you that lion. They have caged him for his preservation; shut him up behind iron bars to secure him from his foes! See how a band of armed men have gathered together to protect the lion. What a clatter they make with their swords and spears! These mighty men are intent upon defending a lion. O fools, and slow of heart! Open that door! Let the lord of the forest come forth free. Who will dare to encounter him? What does he want with your guardian care? Let the pure gospel go forth in all its lion-like majesty, and it will soon clear its own way and ease itself of its adversaries.
::* The Lover of God’s Law Filled with Peace (January 1888)
: and the earlier:
:: There seems to me to have been twice as much done in some ages in defending the Bible as in expounding it, but if the whole of our strength shall henceforth go to the exposition and spreading of it, we may leave it pretty much to defend itself. I do not know whether you see that lion — it is very distinctly before my eyes; a number of persons advance to attack him, while a host of us would defend the grand old monarch, the British Lion, with all our strength. Many suggestions are made and much advice is offered. This weapon is recommended, and the other. Pardon me if I offer a quiet suggestion. Open the door and let the lion out; he will take care of himself. Why, they are gone! He no sooner goes forth in his strength than his assailants flee. The way to meet infidelity is to spread the Bible. The answer to every objection against the Bible is the Bible.
::* Speech at the Annual Meeting of the British and Foreign Bible Society "The Bible" (5 May 1875), in Speeches by C. H. Spurgeon at Home and Abroad (1878) edited by G.H. Pike https://books.google.com/books?id=j_0CAAAAQAAJ&pg=PA17&lpg=PA17#v=onepage&q&f=false
Misattributed

Karel Appel – the complete sculptures,' (1990) not-paged

2. Complexity and Contradiction vs. Simplification or Picturesqueness
Complexity and Contradiction in Architecture (1966)

My Mortal Enemy (1926)

David Colander, Complexity and the History of Economic Thought, Routledge, London and New York, 2000, p. 6.
2000s

This quote was used, and attributed to Jean-Francois Revel, by Jeane Kirkpatrick in her August 20, 1984 speech to the Republican national convention in Dallas, Texas. As cited in Lend Me Your Ears: Great Speeches in History (rev.), ed. William Safire, W. W. Norton & Co. (2004), p. 1029 ISBN 0393059316, 9780393059311
1980s

As quoted in David Crockett : His Life and Adventures (1875) by John Stevens Cabot Abbott, p. 294
Context: I know nothing, by experience, of party discipline. I would rather be a raccoon-dog, and belong to a Negro in the forest, than to belong to any party, further than to do justice to all, and to promote the interests of my country. The time will and must come, when honesty will receive its reward, and when the people of this nation will be brought to a sense of their duty, and will pause and reflect how much it cost us to redeem ourselves from the government of one man.

Foreword http://www.bartleby.com/55/100.html
1910s, Theodore Roosevelt — An Autobiography (1913)
Context: Facing the immense complexity of modern social and industrial conditions, there is need to use freely and unhesitatingly the collective power of all of us; and yet no exercise of collective power will ever avail if the average individual does not keep his or her sense of personal duty, initiative, and responsibility. There is need to develop all the virtues that have the state for their sphere of action; but these virtues are as dust in a windy street unless back of them lie the strong and tender virtues of a family life based on the love of the one man for the one woman and on their joyous and fearless acceptance of their common obligation to the children that are theirs. There must be the keenest sense of duty, and with it must go the joy of living; there must be shame at the thought of shirking the hard work of the world, and at the same time delight in the many-sided beauty of life.