Whatever you believe is your religion

So, this is what I mean by this:

Most people  in the U.S believe in.: Christianity,  democracy, capitalism, Being an American Citizen.

So, one could say that this is the belief system of most Americans who are Christians or Secular Christians is the above. For example, you can be a secular Christian and still go to church or not.

Then you could add to this most common belief system: Science

Also, in the U.S. you might have someone who believes in Atheism, democracy, capitalism, Being an American Citizen and Science but also believes in people in a specific region of the U.S. where their family lives or is from.

I think this is a more useful way to view belief in the world than just using one person's religion. Because each and every thing you believe in is actually a part of your complete belief system.

In the past you might get away with saying you were just a Christian and leave it at that.

You cannot really understand anyone without understanding enough about them. Simply saying someone is a Christian or an Atheist isn't enough.

Demographics is based upon layers of beliefs not just one single identifier.