how is sacrifice a major theme in the novel?

The characters in this story are ridden with guilt and sacrifice to appease their guilt. Tobias feels guilty for his role in the plan that resulted in the explosion that put Uriah into a deathly coma. Tris still feels guilty for killing her friend Will -- so guilty that she has trouble firing a gun. Caleb feels guilty for not defending his sister, choosing to deliver her to what was almost her execution at the hands of Jeanine Matthews. Instead of letting their guilt rule them, these characters learn to live with their guilt. Tobias decides to enter the city to tell Uriah's family about what happened to him. Caleb attempts to sacrifice his own life by entering the Weapons Lab, and Tris (having apologized long ago) tries to move on from her past. Each has their own way of dealing with guilt, but they all attempt to achieve the same goal: to live with this guilt. Often guilt can incapacitate a person, but the characters in this story must keep living and fighting, or else they would lose their lives.