Why do people like Bill O'Reilly, Sarah Palin, George Bush and so on say that America is a christian nation?

Why do people like Bill O'Reilly, Sarah Palin, George Bush, John McCain and so on say that America is a christian nation. The founding fathers of the United States were secularists above all. We know that America is a secular nation. For instance Franklin said that Lighthouses are more useful than Churches. Jefferson said that Christianity is the most perverted religion that has ever shone on man. But why do these people preach America to be a christian nation and why do almost many Americans think America to be a christian nation. It also bothers me that majority of americans can't explain what the bill of rights is nor they can name a single branch of government. 24% can't name the country america fought in the revolutionary war. Half of americans don't even know that states have two senators and most can't name their congressmen. 1/3rd of democrats believed that George Bush had prior knowledge of the 9/11 attack. 18% believe the sun revolves around the earth and 50% believe the earth is flat. The most funny part is that almost 50% of the americans are aware that Judaism is older than Christianity.