“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.”

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

The Zebra
How to Tell Your Friends from the Apes (1931)

Art of Politics (1729). Colonel Titus is reported to have said, "I hope we shall not be wise as the frogs to whom Jupiter gave a stork for their king. To trust expedients with such a king on the throne would be just as wise as if there were a lion in the lobby, and we should vote to let him in and chain him, instead of fastening the door to keep him out". On the Exclusion Bill, Jan. 7, 1681.

Variant: Nature shows us only the tail of the lion. But I do not doubt that the lion belongs to it even though he cannot at once reveal himself because of his enormous size.

As quoted by Abraham Pais in Subtle is the Lord:The Science and Life of Albert Einstein (1982), p. 235 ISBN 0-192-80672-6
Source: Letter to Heinrich Zangger (10 March 1914), quoted in The Curious History of Relativity by Jean Eisenstaedt (2006), p. 126 http://books.google.com/books?id=d2bnXTOtCD8C&lpg=PP1&pg=PA126#v=onepage&q&f=false.

Book II, Chapter 3, "The Shocking Alternative"
Mere Christianity (1952)
Context: I am trying here to prevent anyone saying the really foolish thing that people often say about Him: 'I'm ready to accept Jesus as a great moral teacher, but I don't accept His claim to be God.' That is the one thing we must not say. A man who was merely a man and said the sort of things Jesus said would not be a great moral teacher. He would be either a lunatic—on a level with the man who says he is a poached egg—or else he would be the Devil of Hell. You must make your choice. Either this man was, and is, the Son of God: or else a madman or something worse. You can shut Him up for a fool, you can spit at Him and kill Him as a demon; or you can fall at His feet and call Him Lord and God. But let us not come with any patronising nonsense about His being a great human teacher. He has not left that open to us. He did not intend to.

Source: The Prince (1513), Ch. 18
Variant translations of portions of this passage:
Every one admits how praiseworthy it is in a prince to keep faith, and to live with integrity and not with craft. Nevertheless our experience has been that those princes who have done great things have held good faith of little account, and have known how to circumvent the intellect of men by craft, and in the end have overcome those who have relied on their word.
Ch. 18. Concerning the Way in which Princes should keep Faith (as translated by W. K. Marriott)
A prince being thus obliged to know well how to act as a beast must imitate the fox and the lion, for the lion cannot protect himself from traps, and the fox cannot defend himself from wolves. One must therefore be a fox to recognize traps, and a lion to frighten wolves.
You must know there are two ways of contesting, the one by the law, the other by force; the first method is proper to men, the second to beasts; but because the first is frequently not sufficient, it is necessary to have recourse to the second.
Context: A prince being thus obliged to know well how to act as a beast must imitate the fox and the lion, for the lion cannot protect himself from snares, and the fox cannot defend himself from wolves. One must therefore be a fox to recognise snares, and a lion to frighten wolves. Those that wish to be only lions do not understand this.
Context: How laudable it is for a prince to keep good faith and live with integrity, and not with astuteness, every one knows. Still the experience of our times shows those princes to have done great things who have had little regard for good faith, and have been able by astuteness to confuse men's brains, and who have ultimately overcome those who have made loyalty their foundation. You must know, then, that there are two methods of fighting, the one by law, the other by force: the first method is that of men, the second of beasts; but as the first method is often insufficient, one must have recourse to the second. It is therefore necessary to know well how to use both the beast and the man. This was covertly taught to princes by ancient writers, who relate how Achilles and many others of those princes were given to Chiron the centaur to be brought up, who kept them under his discipline; this system of having for teacher one who was half beast and half man is meant to indicate that a prince must know how to use both natures, and that the one without the other is not durable. A prince being thus obliged to know well how to act as a beast must imitate the fox and the lion, for the lion cannot protect himself from snares, and the fox cannot defend himself from wolves. One must therefore be a fox to recognise snares, and a lion to frighten wolves. Those that wish to be only lions do not understand this. Therefore, a prudent ruler ought not to keep faith when by so doing it would be against his interest, and when the reasons which made him bind himself no longer exist. If men were all good, this precept would not be a good one; but as they are bad, and would not observe their faith with you, so you are not bound to keep faith with them.... those that have been best able to imitate the fox have succeeded best. But it is necessary to be able to disguise this character well, and to be a great feigner and dissembler.

Saddam Hussein, Defiant Dictator Who Ruled Iraq With Violence and Fear, Dies http://www.nytimes.com/2006/12/30/world/middleeast/30saddam.html (The New York Times, 30 December 2006, page A10)
In response to guffaws from a spectator in an overhead gallery during his trial, 2006.

Part II, Ch. XI : Mr. Valiant-For-Truth
The Pilgrim's Progress (1678), Part II

Jacula Prudentum (1651)