The central lesson Harriet Beecher Stowe wanted readers to learn after reading Uncle Tom's Cabin was that slavery was an unjust and un-Christian institution. The latter part of this argument is especially significant since several slave owners and pro-slavery advocates in the South argued that slavery was compatible with Christian morality. By emphasizing the brutality of slaveholders, in particular Simon Legree, who has Tom beaten to death and regularly rapes Cassy, Stowe is showcasing that slavery both victimizes the enslaved and corrupts the morality of the slave owner.

Remarkably for the time, Stowe also shows that the more "benevolent" slave owners and white abolitionists are racist themselves. While they may not beat, rape, or kill slaves, they still view them as subhuman. The hypocrisy of the abolitionist Miss Ophelia is a prime example: she teaches slaves to read but is disgusted at the thought of touching them or connecting with them on an emotional level. She might oppose slavery, but she still refuses to see black people as children of God, as Stowe might put it. Another example would be the St. Clares: Augustine St. Clare sees the moral problems of slavery but feels since he cannot abolish them, he may as well keep owning slaves, especially since he makes a lot of money doing so. His passivity perpetuates injustice, making him in a way responsible for Tom's eventual murder at the hands of Tom's final owner, Simon Legree.

Within the book itself, the message of Uncle Tom's Cabin is clear. Slavery was fundamentally evil, corrupting everyone it touched and destroying the lives of good, pious men like Uncle Tom. The point of the book was to emphasize the horrors of slavery—its author, Harriet Beecher Stowe, was an abolitionist. Stowe's objections to slavery were based on her Christianity, and slavery in the book is in many ways the antithesis of Christian beliefs. Stowe consistently points out the hypocrisy of Christian slaveholders, and Uncle Tom's Cabin is full of biblical references that would have been readily understood by her readers. For example, after a chapter describing the New Orleans slave warehouses, and an auction that took place in one, she quotes Psalms 9:12 to warn that slaveholders, and indeed the country as a whole, faced divine punishment for the sin of slavery: "When he maketh inquisition for blood, he forgetteth not the cry of the humble!"

Read in context, Uncle Tom's Cabin offers another lesson for readers. It is an example of the power of literature and art to shape public opinion. Published in 1852, the book both played on Northern discontent with the Fugitive Slave Act of 1852 and intensified this public perception of the incident. It is probably not true that Abraham Lincoln described Stowe as the "little lady who started the big war," but certainly her book dramatized the horrors of slavery for Northerners.

Uncle Tom's Cabin emphasizes the abolitionist stance that slavery under any condition is torturous and evil and that an unthinkable cruelty is committed against people when you claim ownership over their lives. While the Shelbys are depicted as kindhearted, the reality that they choose to withhold freedom from another person brings them in direct conflict with this depiction. Regardless of their demeanors, when the Shelbys decide it benefits them, they rip apart Tom's family and sell him to a particularly evil slave-owner who eventually tortures Tom to death. Harriet Beecher Stowe underscores that one cannot be a good person as well as a slave owner and that there is no ethical method of owning another person. As white Americans read Uncle Tom's Cabin during the times of plantation slavery, they would have hopefully learned the lesson that slavery cannot be reformed into an acceptable institution and that the fundamental principles of slavery are unthinkably cruel. The only moral course of action is complete abolition.

The overriding message of Uncle Tom's Cabin is a simple one: slavery is wrong, period. As a slave, it doesn't matter if you have a kind, considerate owner; your life is still not your own, and that can never be a good thing. In presenting the evils of slavery, Stowe cuts right to the heart of the matter. Slavery is to be evaluated as an institution in itself, irrespective of the individuals who own and trade in slaves. And that institution is intrinsically evil because it is based upon an evil principle: that it is acceptable to keep someone in a position of permanent subjection simply on account of their skin color. Some of Tom's owners may be good people in certain respects, but by their continued ownership of slaves, they perpetuate an evil institution, one that is eventually responsible for Tom's brutal, shocking death.

The overarching lesson Stowe wanted to convey in her novel is that even under the best of conditions, slavery was an evil institution. People in her time often justified slavery with the argument that while some owners mistreated their slaves, most were good-hearted and cared for their slaves well. Stowe opens the novel with a depiction of the Shelby family in Kentucky, who are, indeed, good slave owners. Nevertheless, Stowe wants readers to understand that even in the best of situations, slavery is a terrible condition in which to live. Once Mr. Shelby finds himself in financial trouble, he sells his slave Uncle Tom to settle his debts: this shows that any slave, at any time, no matter how good (and Uncle Tom is the best of human beings) could be separated from his wife and children and have no recourse. Uncle Tom eventually dies at the hands of a sociopathic owner. Stowe wants us to take away the lessons that slavery is a cruel, dehumanizing institution, slaves are human beings, and all humans should be treated with decency.