“If a lion could speak, it would not understand itself.”

Source: Black: The Birth of Evil

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

Of Laws.
Political, Moral, and Miscellaneous Reflections (1750), Political Thoughts and Reflections

AppendiX, "A note from Dr. V to Dr. Simpkin"
City of Saints and Madmen (2001–2004)

As quoted in "The Encyclopedist’s Lair" in The New York Times (19 November 2007)
Context: There’s plenty of rude stuff online. People say things online that they would be ashamed to say face to face. If people could treat others as though they were speaking face to face, that would be huge.

Did Adam have a Bellybutton?: And other tough questions about the Bible (2000)

Source: The Monkey Grammarian (1974), Ch. 4
Ch. 4 -->
Context: Fixity is always momentary. But how can it always be so? If it were, it would not be momentary — or would not be fixity. What did I mean by that phrase? I probably had in mind the opposition between motion and motionlessness, an opposition that the adverb always designates as continual and universal: it embraces all of time and applies to every circumstance. My phrase tends to dissolve this opposition and hence represents a sly violation of the principle of identity. I say “sly” because I chose the word momentary as an adjectival qualifier of fixity in order to tone down the violence of the contrast between movement and motionlessness. A little rhetorical trick intended to give an air of plausibility to my violation of the rules of logic. The relations between rhetoric and ethics are disturbing: the ease with which language can be twisted is worrisome, and the fact that our minds accept these perverse games so docilely is no less cause for concern. We ought to subject language to a diet of bread and water if we wish to keep it from being corrupted and from corrupting us. (The trouble is that a-diet-of-bread-and-water is a figurative expression, as is the-corruption-of-language-and-its-contagions.) It is necessary to unweave (another metaphor) even the simplest phrases in order to determine what it is that they contain (more figurative expressions) and what they are made of and how (what is language made of? and most important of all, is it already made, or is it something that is perpetually in the making?). Unweave the verbal fabric: reality will appear. (Two metaphors.) Can reality be the reverse of the fabric, the reverse of metaphor — that which is on the other side of language? (Language has no reverse, no opposite faces, no right or wrong side.) Perhaps reality too is a metaphor (of what and/or of whom?). Perhaps things are not things but words: metaphors, words for other things. With whom and of what do word-things speak? (This page is a sack of word-things.) It may be that, like things which speak to themselves in their language of things, language does not speak of things or of the world: it may speak only of itself and to itself.

The Philosophy of Paine (1925)
Context: Many a person who could not comprehend Rousseau, and would be puzzled by Montesquieu, could understand Paine as an open book. He wrote with a clarity, a sharpness of outline and exactness of speech that even a schoolboy should be able to grasp. There is nothing false, little that is subtle, and an impressive lack of the negative in Paine. He literally cried to his reader for a comprehending hour, and then filled that hour with such sagacious reasoning as we find surpassed nowhere else in American letters — seldom in any school of writing.