“All men would be tyrants if they could.”

Letter to John Adams (31 March 1776), published in Familiar Letters of John Adams and his wife Abigail Adams (1875) edited by Charles Francis Adams, p. 147
Context: I long to hear that you have declared an independency. And by the way, in the the new Code of Laws which I suppose it will be necessary for you to make I desire you would Remember the Ladies, and be more generous and favourable to them than your ancestors. Do not put such unlimited power into the hands of the Husbands. Remember all Men would be tyrants if they could. If particular care and attention is not paid to the Ladies we are determined to foment a Rebellion, and will not hold ourselves bound by any Laws in which we have no voice, or Representation.
That your Sex are Naturally Tyrannical is a Truth so thoroughly established as to admit of no dispute, but such of you as wish to be happy willingly give up the harsh title of Master for the more tender and endearing one of Friend. Why, then, not put it out of the power of the vicious and the Lawless to use us with cruelty and indignity with impunity? Men of Sense in all Ages abhor those customs which treat us only as the vassals of your sex; regard us then as Beings placed by Providence under your protection, and in imitation of the Supreme Being make use of that power only for our happiness.

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

Nous distinguerons dans Robespierre deux hommes apôtre de la liberté et Robespierre le plus infâme des tyrans.
[in Gracchus Babeuf avec les Egaux, Jean-Marc Shiappa, Les éditions ouvrières, 1991, 79, 27082 2892-7, ; Tribun du peuple n°2 du 17 fructidor an II (3 septembre 1794)]
On Maximilien de Robespierre

Vol. I, p. 188
William Lloyd Garrison 1805-1879 (1885)

Source: The Friends of Voltaire (1906), Ch. 8 : Turgot: The Statesman, p. 207

The Conspiracy of Kings (1792)
Context: Lords of themselves and leaders of mankind. On equal rights their base of empire lies,
On walls of wisdom see the structure rise;
Wide o'er the gazing world it towers sublime,
A modell'd form for each surrounding clime.
To useful toils they bend their noblest aim,
Make patriot views and moral views the same,
Renounce the wish of war, bid conquest cease,
Invite all men to happiness and peace,
To faith and justice rear the youthful race,
Till Truth's blest banners, o'er the regions hurl'd,
Shake tyrants from their thrones, and cheer the waking world.

Source: A Thousand & One Epigrams: Selected from the Writings of Elbert Hubbard (1911), p. 13.

1860s, Letter to James C. Conkling (1863)
Context: But, to be plain, you are dissatisfied with me about the negro. Quite likely there is a difference of opinion between you and myself upon that subject. I certainly wish that all men could be free, while I suppose you do not. Yet I have neither adopted, nor proposed any measure, which is not consistent with even your view, provided you are for the Union. I suggested compensated emancipation; to which you replied you wished not to be taxed to buy negroes. But I had not asked you to be taxed to buy negroes, except in such way, as to save you from greater taxation to save the Union exclusively by other means.

Aristippus, 4.
The Lives and Opinions of Eminent Philosophers (c. 200 A.D.), Book 2: Socrates, his predecessors and followers