I think literally 9 or 10 of the 11 official declarations of secession mentioned slavery explicitly, to say nothing of the public statements by their founding traitors, to wit:

The prevailing ideas entertained by [Jefferson] and most of the leading statesmen at the time of the formation of the old constitution, were that the enslavement of the African was in violation of the laws of nature; that it was wrong in principle, socially, morally, and politically. It was an evil they knew not well how to deal with, but the general opinion of the men of that day was that, somehow or other in the order of Providence, the institution would be evanescent and pass away.

Our new government is founded upon exactly the opposite idea; its foundations are laid, its corner- stone rests, upon the great truth that the negro is not equal to the white man; that slavery subordination to the superior race is his natural and normal condition. This, our new government, is the first, in the history of the world, based upon this great physical, philosophical, and moral truth. This truth has been slow in the process of its development, like all other truths in the various departments of science. It has been so even amongst us. Many who hear me, perhaps, can recollect well, that this truth was not generally admitted, even within their day. The errors of the past generation still clung to many as late as twenty years ago.

Motherfucker actually argued with a straight face that race-based chattel slavery was good and right and progressive.