Three main themes in this book are illustrated in the lives of three main characters. Huck exemplifies conscience, Jim demonstrates freedom, and Tom represents racism.
Huck's character demonstrates conscience with nearly every decision. Throughout the beginning of the novel, readers see his morality in decision-making. He longs to be free from the grip of his father's abuse, but also the rigid ritualistic religion of Miss Watson. Neither extreme provides fruitful living for Huck. Both provide condemnation or pain. Therefore, he escapes with the inner struggle that he has had to make people believe he is dead. His most difficult struggle with conscience comes with choosing to turn in Jim or not. When some folks are looking for escaped Negroes, it would be easy to turn Jim in. He struggles with the types of right in the world: the morally right, the humanely right, and the legally right. He makes a humane decision to keep Jim and help him find freedom at the risk of going to hell for not doing what is legally right (or religiously right).
Jim pursues freedom through the entire novel. Even though the process means he is at times kept in the wigwam like a prisoner, or dressed like an Arab, Jim is willing to endure great pain and trial in order to gain liberty. It seems throughout the text that Twain tries to demonstrate the problem and inhumanity of slavery. The relationship between Jim and Huck grows and demonstrates what a poor black man and a sharper white boy can teach each other. The final binding of Jim in the Phelps' shed coupled with the revelation that Jim is indeed a free man illustrates the concept that the one human having the capacity to inflict bondage upon another is not acceptable. Twain's theme positions readers to think about how they have infringed upon others in their own lives.
Tom Sawyer represents racism, although that doesn't make him a racist character. Twain stays away from demonstrating racism in characters, and lets readers draw their own conclusions. Tom's efforts to keep Jim bound for the glory of the eventual escape that he had planned makes the reader consider why people have been separated because of color. Tom doesn't necessarily see the color of Jim, he sees the opportunity. Although this concept is displayed through childhood imagination, it can be applied to the cruel treatment adults placed on other adults purposely during slavery.
Other connections can easily be drawn, but these are some of the broader reaching themes. Imagination, childhood, education, learning, and religion are all themes Twain also displays.