"Selah" is a Hebrew word. As Mr. Small tells Thomas, it means "raise the voice" (2.10). It's the name slaves gave to Dies Drear because he helped them raise their voices to find freedom.
And they returned to bondage hoping to free masses of slaves. (2.7)
Thomas is surprised by this piece of information – that slaves would voluntarily go back into slavery to help save others. It makes sense, though, doesn't it? Slavery divides families, friends, and loved ones from each other. It's natural that those who got away and found help would want to go back and help others, no matter the risk.
Thomas felt frantically along the wall. The wall was wood. […] Thomas pounded on it, hurting himself more, causing his head to spin. He kept on, because he knew he was about to be taken from behind by something ghostly and cold. (4.5)
In "Genre" we say this is Gothic literature. One of the biggest guys in Gothic fiction is Edgar Allan Poe. This moment seems right out of Poe. He's known for putting his characters in tight spaces. But, unlike many trapped Poe characters, Thomas finds his way out to freedom.
"It sure does feel good to be here. It sure feels like home." (10.10)
Even though he's nervous and the people are less than friendly, Thomas immediately feels at home in the new church. Feeling at home is a feeling of freedom.
"I take freedom any day over that romantic nonsense about slavery." (15.29)
In this moment at least, Mayhew feels that it's more important to embrace freedom in the present, than to dwell on slavery in the past. The novel as a whole celebrates the struggle for freedom that makes the present freedom celebration possible. (Now say that last sentence ten times in a row, fast as you can.)
"He would have to find the past to find himself." (17.11)
This is Mayhew talking about his father. Now that he's cooled down he shows that he believes that knowledge of the past can be freeing. Finding the treasure gave him a way to understand the past of his ancestors. He now better understands how he came to be where he is today. Being able to trust others with his secret gives him another level of freedom.
I have lived these caves for fifty years. I have lived them when no one cared but the damp and the dark. And now you come here telling me how to be and how to die. I'll not leave. I'll not go to any hospital. (17.1)
Here we see that Mr. Skinner has found freedom in the cave. He's free because he's independent. He's found some kinship with the environment of the cave. But, it's also been very lonely. When Mayhew and the Smalls show they care, but respect his wishes, he begins to trust again, and he has more than "the damp and the dark" to rely on.
[…] he began to feel as though he were a slave hiding and running. Somewhere in the back of his mind was emptiness and fear; loneliness, the way a desperate slave would feel. (18.30)
Waiting in the dark for the Darrows, Mr. Small identifies with the plight of the runaway slaves who had once hid in the same place. Of course, those people were in infinitely more dangerous and desperate situations. What he experiences is a tiny fraction of what a person running from slavery would.