I think that one of the strongest manners in which Stowe is able to bring out the humanity of slaves to her readers is through the use of religion.  Stowe is mindful of how slaves are seen as property.  The notion of slaves as being objects or property helped to continue the dehumanization and degradation that was intrinsic to slavery.  Stowe is able to remove this through her understanding of religion as part of this process.  The fact that Tom is able to tell Legree that, despite cruelty and violence, he is loyal to something more transcendent is a part of this humanizing process:

My soul an't yours, Mas'r!….It's been bought and paid for by one that is able to keep it…

Stowe is able to reveal the humanity of slaves in this particular moment in showing that slaves were able to believe in a transcendent form of spiritual worship.  This embodies the basic tenet of Christianity, the dominant religion of the time period, in that what happens to individuals in this life is secondary to being embraced by the savior in the afterlife.  In doing so, Stowe is able to reveal and illuminate the humanity of slaves, seeing them as more than property and actually revealing them to possess dignity and compassion.