“It is much to be wished that slavery may be abolished. The honour of the States, as well as justice and humanity, in my opinion, loudly call upon them to emancipate these unhappy people. To contend for our own liberty, and to deny that blessing to others, involves an inconsistency not to be excused.”

Source: Christianity and the Social Crisis (1907), Ch.4 Why Has Christianity Never Undertaken the Work of Social Reconstruction?, p. 150

Yorktown Oration (1881).

As quoted in His Brother's Blood: Speeches and Writings, 1838–64 https://books.google.com/books?id=qMEv8DNXVbIC&pg=PA193&lpg=PA199 (2004), edited by William Frederick Moore and Jane Ann Moore, p. 199
1860s, Speech to the U.S. House of Representatives (April 1860)

1780s, Letter to Reverend Doctor Price (1785)

1860s, The Constitution of the United States: Is It Pro-Slavery or Anti-Slavery? (1860)
Context: I, on the other hand, deny that the Constitution guarantees the right to hold property in man, and believe that the way to abolish slavery in America is to vote such men into power as well use their powers for the abolition of slavery. This is the issue plainly stated, and you shall judge between us.

From ‘A Duty to Posterity’, as contained in A Library of American Literature From the Earliest Settlement to the Present Time, Volume 3, ed. Edmund Clarence Stedman, C. L. Webster (1892), pp. 177-178

2000s, 2003, Hope and Conscience Will Not Be Silenced (July 2003)
Context: By a plan known only to Providence, the stolen sons and daughters of Africa helped to awaken the conscience of America. The very people traded into slavery helped to set America free. My Nation's journey toward justice has not been easy, and it is not over. The racial bigotry fed by slavery did not end with slavery or with segregation. And many of the issues that still trouble America have roots in the bitter experience of other times. But however long the journey, our destination is set: liberty and justice for all.

Source: 1990s, For Cause and Comrades: Why Men Fought in the Civil War (1997), p. 106
Context: These soldiers were using the word slavery in the same way that Americans in 1776 had used it to describe their subordination to Britain. Unlike many slaveholders in the age of Thomas Jefferson, Confederate soldiers from slaveholding families expressed no feelings of embarrassment or inconsistency in fighting for their own liberty while holding other people in slavery. Indeed, white supremacy and the right of property in slaves were at the core of the ideology for which Confederate soldiers fought.

A Dissertation on Slavery: With a Proposal for the Gradual Abolition of it, in the State of Virginia (1796)