“I assert most unhesitatingly, that the religion of the South — as I have observed it and proved it — is a mere covering for the most horrid crimes; a justifier of the most appalling barbarity; a sanctifier of the most hateful frauds; and a dark shelter, under which the darkest, foulest, grossest, and most infernal abominations fester and flourish. Were I again to be reduced to the condition of A slave, next to that calamity, I should regard the fact of being the slave of a religious slaveholder, the greatest that could befall me.”

Letter to John Adams (24 September 1774)
Context: I wish most sincerely there was not a slave in this province. It always appeared a most iniquitous scheme to me — to fight ourselves for what we are daily robbing and plundering from those who have as good a right to freedom as we have.

Act I, sc. i.
The Critic (1779)

Amor servo de l'oro, è il maggior mostro,
Et il più abominabile, e il più sozzo,
Che produca la terra, o 'l mar frà l'onde.
Act II, scene i.
Aminta (1573)

1860s, The Constitution of the United States: Is It Pro-Slavery or Anti-Slavery? (1860)
Context: My argument against the dissolution of the American Union is this. It would place the slave system more exclusively under the control of the slave-holding states, and withdraw it from the power in the northern states which is opposed to slavery. Slavery is essentially barbarous in its character. It, above all things else, dreads the presence of an advanced civilization. It flourishes best where it meets no reproving frowns, and hears no condemning voices. While in the Union it will meet with both. Its hope of life, in the last resort, is to get out of the Union. I am, therefore, for drawing the bond of the Union more completely under the power of the free states. What they most dread, that I most desire. I have much confidence in the instincts of the slaveholders. They see that the Constitution will afford slavery no protection when it shall cease to be administered by slaveholders. They see, moreover, that if there is once a will in the people of America to abolish slavery, this is no word, no syllable in the Constitution to forbid that result. They see that the Constitution has not saved slavery in Rhode Island, in Connecticut, in New York, or Pennsylvania; that the Free States have only added three to their original number. There were twelve Slave States at the beginning of the Government: there are fifteen now.

Source: Memoirs, Unreliable Memoirs (1980), p. 64

Source: 1790s, The Age of Reason, Part II (1795), Chapter III: Conclusion.

Introduction, pp. 3-4.
The Causes of Evolution (1932)

Introduction
An Enquiry into the Obligations of Christians (1792)
Context: In one period the grossest ignorance and barbarism prevailed in the world; and afterwards, in a more enlightened age, the most daring infidelity, and contempt of God; so that the world which was once over-run with ignorance, now by wisdom knew not God, but changed the glory of the incorruptible God as much as in the most barbarous ages, into an image made like to corruptible man, and to birds, and four-footed beasts, and creeping things. Nay, as they increased in science and politeness, they ran into more abundant and extravagant idolatries.

Source: Narrative of the Life of Frederick Douglass