Why do christians think the u.s. is a christian nation when we are the first secualar one?

Most of the founding fathers were Deists by the way. Not christians. That includes Washington, Franklin, Adams, and Jefferson. This isn't and never was a christian nation. It was founded as a nation of free thought. We have separation of church and state for a reason. Because the founding fathers were aware of how destructive it was. Look at where they came from. Europe tore itself apart for centuries because everyone wanted their religion to be the "right one"