The theme of trust is definitely present in this visceral novel. Whether or not it is the main contributing factor to the survival of the man and the boy is up to individual readers. The following quote shows trust between the two characters and trust in a goal.

He said that everything depended on reaching the coast, yet waking in the night he knew that all of this was empty and no substance to it. There was a good chance they would die in the mountains and that would be that.

This quote shows that on some part, the man does hope and trust that things will be better on the coast. The boy follows his father, and that shows the trust that the boy has in his father.

One of the best trust quotes in my opinion is the following quote. It is a very brief exchange between the man and the boy, but it speaks volumes about trust.

"Do you think I lie to you?"
"No."
"But you think I might lie to you about dying."
"Yes."
"Okay. I might. But we’re not dying."
"Okay."

The conversations between the man and the boy are always short and clipped like this. The boy usually responds with single word answers. He speaks his honest thoughts and doesn't argue his father's points and conclusions. I believe this shows a great deal of trust between the two individuals. The man's honesty toward his son also shows that he trusts his son to know some fairly difficult and disheartening information.

I think that trust in one another becomes a major reason why the boy and his father are able to endure and survive in the difficult conditions of a post- apocalyptic world in which they live and sojourn.  It becomes evident throughout their struggles that the only driving force that galvanizes them to continue is trust in one another.  We can see this in an exchange when the father defends his son:

You wanted to know what the bad guys looked like. Now you know. It may happen again. My job is to take care of you. I was appointed to do that by God. I will kill anyone who touches you. Do you understand?

Yes.

He sat there cowled in the blanket. After a while he looked up. Are we still the good guys? he said.

Yes. We’re still the good guys.

Consider the implications of this passage and exchange.  The fact that the father and son trust one another in being "the good guys" in a setting where there are few is one reason trust enables them to survive.  At the same time, I think that it is trust that compels the son to believe in the father and the father to defend his son by any means necessary.  When the father says, "I will kill anyone who touches you," it is an instant where trust is felt and experienced by both the father who recognizes the importance of the bond between both and the son who understands how much the father trusts him and their relationship amidst a world that has severed as many bonds of trust as possible.