In America EVERYONE is free to express THEIR OWN religious beliefs or lack thereof.
For that is why many fled to the New World in the first place to escape the religious control and persecution by The Church of England that had so intertwined itself with government that it actually was a huge part of their government.
The Founders wanted no part of that ever repeating in America. Many of the Founders weren't even Christians but Deists who believed that there was a Creator a "God" but that God only started the basic elements of life and then disappeared with no further influence on the outcome nor heaven or hell. No relatives of God... no Saints... no Prophets. So it's obvious with such diversity of agreement/disagreement (in this plus all of the hundreds of other religions of the world) the whole "God thing" is totally subjective.
It's much like people being burned at the stake for being witches. There was a time when people obviously believed supernaturally in witches whole heartedly. But through the developing of science & education humans grew past those more primitive beliefs.
And when you look at the science of things, carbon dating, geology, the proven evolution of species and the actual way religion spread, all the thousands of completely different religions and the gods contained therein, the locations of doctrine, the copying of stories of previous & completely different religious beliefs there is absolutely no scientific or factual basis to believe religion to be true. It's ALL a faith in something unprovable. To an unbiased person (a person not indoctrinated into a particular mindset) religion appears to be nothing more than a coping mechanism to help explain the unexplainable to primitive man.
So in conclusion to say, prove God doesn't exist (the intentionally impossible task of proving a super natural negative) is the exact same thing as me saying... prove OldTrapper isn't a witch, or prove fairies or leprechauns don't exist.
The fact is the burden of proof is on the one wanting to make the claim that something DOES exist and not the other way around. And in the case of a god that simply cannot be done.
