“Men must be governed by God or they will be ruled by tyrants.”
This has been quoted as Penn's in various forms since at least 1943 (Fulton J. Sheen, Philosophies at War, p. 154). James H Billington of the Library of Congress wrote (Respectfully Quoted: A Dictionary of Quotations, 2010, p. 145) "Numerous sources cite this remark but it has not been found in Penn's writings." Other variants include:
Unless we are governed by God, we shall be ruled by tyrants. (1949 speech by Norman Vincent Peale)
If men do not find God to rule them, they will be ruled by tyrants. (Roy Masters, How to Conquer Suffering Without Doctors, 1976, p. 50)
... those who will not be governed by God will be ruled by tyrants. (David Barton, The Myth of Separation, 1992, p. 89
Misattributed
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William Penn 53
English real estate entrepreneur, philosopher, early Quaker… 1644–1718Related quotes

“The tyrant dies and his rule is over; the martyr dies and his rule begins.”
1848
1840s, The Journals of Søren Kierkegaard, 1840s
Source: The Journals of Kierkegaard
"Liberal Education: Enabling Citizens to do their Duty as Free Men", Report of the president of St. John's College to the Board of Visitors and Governors, May, 1941. Hanging in Buchanan Hall, St. John's College Annapolis

1990s, The End of History Means the End of Freedom (1990)

Source: Civil Government : Its Origin, Mission, and Destiny (1889), p. 73
Context: Human government, the embodied effort of man to rule the world without God, ruled over by "the prince of this world," the devil. Its mission is to execute wrath and vengeance here on earth. Human government bears the same relation to hell as the church bears to heaven.

The Farmer Refuted (1775)
Context: The origin of all civil government, justly established, must be a voluntary compact, between the rulers and the ruled; and must be liable to such limitations, as are necessary for the security of the absolute rights of the latter; for what original title can any man or set of men have, to govern others, except their own consent? To usurp dominion over a people, in their own despite, or to grasp at a more extensive power than they are willing to entrust, is to violate that law of nature, which gives every man a right to his personal liberty; and can, therefore, confer no obligation to obedience.

As cited in: Richard Mann Roberts, Carlo Pisacane's La Rivoluzione, Troubador, 2010, p. 160

Letter to Peter the Great, the Czar of Russia, 2 July 1698, in Samuel McPherson Janney, The Life of William Penn (Philadelphia, 1852), p. 407

“Governments throughout history have been run by tyrants.”
The Tyrant Next Time (November 7, 2019)