“None are more hopelessly enslaved than those who falsely believe they are free.”

The Tragic Sense of Life (1913), IV : The Essence of Catholicism
Context: ... as the great Unitarian preacher Channing pointed out, that in France and Spain there are multitudes who have proceeded from rejecting Popery to absolute atheism, because "the fact is, that false and absurd doctrines, when exposed, have a natural tendency to beget skepticism in those who receive them without reflection. None are so likely to believe too little as those who have begun by believing too much." Here is, indeed, the terrible danger of believing too much. But no! the terrible danger comes from another quarter — from seeking to believe with the reason and not with the life.

1770s, African Slavery in America (March 1775)

Don, in The Philanthropist (1969), scene 6

Speech https://books.google.com/books?id=HGM9AQAAMAAJ&pg=PA5172 (1870)

Source: The Count of Monte Cristo

Letter From Thomas Jefferson to the Rev. James Madison, 19 July 1788
1780s

Context: I do not think idolatry the worst of sins. Cruelty is the worst of sins. It is far better to worship a false God, than to injure your neighbor—far better to bow before a monstrosity of stone, than to enslave your fellow-men.

Part 3, Section 2
A Treatise of Human Nature (1739-40), Book 2: Of the passions