Chief Justice Taney, who wrote the Dred Scott decision, stated that although his personal religious views (he was Catholic) were against slavery (he freed all those he had inherited, and gave the old ones pensions), he couldn't impose those views on everyone.
This is an issue of Constitutional Law and should be reserved for the politics forum.
Shouldn't be hard at all. Thomas Jefferson was a son of the South, the father of secularism here, and quite the Renaissance thinker where he wrote that blacks were naturally inferior to whites (an Enlightenment idea, btw), and smelt bad.
The issue with Thomas Jefferson is MUCH more complex than the simplistic picture you are trying to paing. While he continued to hold slaves but never supported the institution, his acts and views may have been a bit hypocritical, but he did, at least, understand the ideals of the Enlightenment in theory. This hypocracy troubled him all his life and the pains of his conscience can clearly be seen in his writings, he wanted to free his slaves but was deeply in debt and could not do so as they were named as collateral for notes and mortgages, he desired to free his slaves once he had paid these notes, but with the decrease of land values in 1819 this would never be possible for him.
In 1769 he drafted legislation that was presented to the Virgina House of Burgesses to emancipate all slaves, but it was ultimately defeated. In his original draft of the declaration of independence he included words of condemnation for the Crown saying, 'charging that the crown "has waged cruel war against human nature itself, violating its most sacred rights of life and liberty in the persons of a distant people who never offended him, captivating & carrying them into slavery in another hemisphere.' But these words were removed at the insistence of delegates from South Carolina and Georgia. Due to his political activities, Virgina, whose economy was heavily dependent on slavery, became the first American state to ban the importing of slaves into the state, commenting on the legislation he said that it 'stopped the increase of the evil by importation, leaving to future efforts its final eradication.' In 1807, when it became constitutionally premissible to pass such a law, he signed the bill that outlawed the importing of slaves into the United States.
In his September 10, 1814 letter to Thomas Cooper he clearly laid out his political and philosophical views on the institution of slavery, when he said 'There is nothing I would not sacrifice to a practicable plan of abolishing every vestige of this moral and political depravity.'
In his well-known work, Notes on the State of Virginia
, he said, 'There must doubtless be an unhappy influence on the manners of our people produced by the existence of slavery among us. The whole commerce between master and slave is a perpetual exercise of the most boisterous passions, the most unremitting despotism on the one part, and degrading submissions on the other.'
And while he did not view whites and blacks to be equals in matters of intelligence or capability (and, thus, did not believe that they could peacefully live together as equals in a common society), he did believe that they were equal in nature and that slavery was immoral.
What we see in Jefferson is not a condoning of slavery by enlightenment ideology, but rather the epitome of the traditional conflict between philosophical ideals and practical economics.
As for the non-religious vs. religious, the original and uncompromising Abolititionist movements all grew out of churches.
Actually the divide was between liberal and conservative Christianity. Liberal Christianity, which had been heavily influenced by the Enlightenment, had become anti-slavery; conservative Christianity condemned these liberals for incorporating secular ideals into their theology and going directly against both the Old and New Testaments as well as Christian tradition. I would recommend Dr. Dabney's A Defence of Virginia and the South
if you actually want to read about this theological controversy, though the book is clearly from the conservative side of the debate.