Survival versus humanity
The father does whatever it takes to keep the boy alive. The boy keeps asking whether they are still good people. The novel puts these two things in constant tension. Staying alive and staying human are not always the same thing.
A parent's love as the last meaning
The father has no religion, no community, no future. The boy is the only reason he keeps moving. His love for the boy is the one thing that gives his life structure, and it is also what makes every threat feel unbearable.
The collapse of civilization
The world in the novel is not just ruined physically. Social trust is gone. Language is stripped down. People prey on each other. McCarthy shows what happens when the systems that hold society together disappear completely.
Memory and loss
The father carries memories of his wife and the old world. Those memories comfort him but also remind him of everything that is gone. At some point he has to let them go to keep moving. The past becomes a weight.
Good versus evil without institutions
Without laws, governments, or religion to enforce moral codes, the characters have to decide for themselves what good means. The father and boy call themselves the good guys. The novel asks what that label actually requires.