If civilization where to crumble because of man made or natural occurrences (it will happen one day), then people who survive will look towards whatever gives them hope, and in many cases that will be religion.
Religion has been used throughout human history to explain the unexplainable and more importantly to control the masses. The best way to create order in a lawless society is to instill fear of the unknown into the people. History shows us that people can be manipulated through fear to do anything. The Crusades, and the modern Islamic evil are examples of the power of moving the masses via a fear of "god".
Based on that, there is no question that religion will be a powerful force, but it will be manufactured as it has in the past a means to control rather than being something people naturally gravitate towards. Mandated religion if you will.
In a barren, hostile landscape without the luxury we have today, many people who spend their two hours in church every Sunday and Wednesday will figure out that God does not get involved and their history of playing dress up and tea party every Sunday made no difference. It will be a bitter pill to swallow for most.
