In Cormac McCarth's novel The Road, I believe that carrying the fire, while it may refer literally to fire, also may refer to the "keepers of the fire," those who historically were able to survive because they had the means to make or carry fire; the carrying of the fire is a metaphor.
Fire was historically used for many things: the first most important uses would have been to keep warm and keep wild animals away. As people advanced and were more sophisticated, fire not only cooked food, but enabled people to work with natural materials to make weapons, crockery and try new foods.
The true success of early man with regard to fire was not just to be able to collect it and protect it when it occurred naturally, say from a lightning strike, but to use materials that would create fire whenever they wanted it. These were the people that survived, procreated, and moved on to populate new areas.
When I think of fire in this book, I see the father and son as "good guys" (as they call themselves): survivors who want not just to get by, but start a new life. They keep the ideals of the old society, the concept of civilized behavior, alive, while other members of society have turned to violence and cannibalism.
This sense of maintaining the course of civilization is seen with the father speaking to his son about surviving, but in an honorable way.
Are we still the good guys? he said.
Yes. We’re still the good guys.
And we always will be.
Yes. We always will be.
Fire can destroy, as seen with the results of the apocolyptic fallout of the world's destruction. Fire can be referred to when they build a fire and stay close for warmth during the cold brought on by the nuclear winter. However, figuratively, I believe "fire" refers to carrying on the age-old goal of survival by man, and doing what they can to rebuild the world in a positive way, not by hurting others and stealing from them.
Acknowledgement of fire tied to goodness is seen in the quote:
"We're the good guys...and we're carrying the fire."
This is a new way seeing life for the father (who has seen the days before the disaster), and something he passes on to his son (who never knew any world but this one). Even after the father dies, we can hope that family that takes the son in will continue the father's quest to find goodness in humanity and rebuild the world. (And based upon the son's desire to help others, we can be comfortably sure that he will do so.)
"Fire" is mentioned 76 times in the novel. Most of the time it is used literally to mean fire. A few times is it used as a metaphor by the father and/or son.
Pg. 83: "Nothing bad is going to happen to us...because we're carrying the fire."
Pg. 129: "We're the good guys...and we're carrying the fire."
Pg. 172: "I havent seen a fire in a long time, thats all. I live like an animal."
Pg. 186: "Nowhere to build a fire. Nowhere safe."
Pg. 216: "Are they carrying the fire too? They could be yes. But we dont know. So we have to be vigilant."
Possible uses as a symbol:
As metaphor, it is fire: that which is used for survival, warmth, cooking, protection. Will the gun "fire"?
Ironically, the earth seems to have been destroyed by fire, so it could symbolize the apocalypse.
It could symbolize death. If the truck people or cannibals see their fire, they could be killed. "Firing" the gun kills the one trucker. It could also end the father's and son's life--as it did the mother's--in suicide.
Metaphorically, it is hope, faith, God, the past, the future, humanity, survival, goodness, family.