It is, in a way. But the fact this country was founded believing in God, our coins have it, etc. it is not exclusionary, as this belief in God was set right when we were founded. Atheists know this, and they cannot do anything about it. They cannot object, as if they object, they therefore object to the way we were founded.
Could you explain how a belief can be 'set'? An individual can have beliefs, but a belief can't be 'set' for others, if that's what you mean.