The answer revolves around the contrast between genuine Christianity and a sham version of it.
Indeed, the Shelbys view themselves as righteous Christians, taking pride in their supposedly humane treatment of their slaves. However, when financial difficulties arise, they resort to selling Uncle Tom, who has cared for Arthur Shelby throughout his life, to a merciless slave trader. They also opt to sell Eliza’s only remaining son to settle their debts.
As a result, they exhibit not only ingratitude and selfishness toward Uncle Tom but also heartlessness toward Eliza, all while considering themselves to be “good Christians.” Furthermore, there's Tom Loker, a Southerner who identifies as a Christian yet makes his living by tracking down black people as though they were game. His inauthentic Christianity becomes apparent when he is harbored and treated after being wounded by Quakers, who assist runaway slaves on their journey to Canada and show kindness toward him, though he has caused considerable pain and death among countless slaves.
While recuperating, he comes to understand the immorality of his previous actions and ultimately reforms by joining the underground railroad to aid escaping slaves; thus, he makes amends for his earlier wrongdoings.
