There is a legal reason we need God in the US. I think it startles patriotic atheists to hear it, but most atheists seem gleeful to know it.
The Bill of Rights is a list of our natural rights, given us by Our Creator, and they precede and pre-date our government. They are listed in the Bill of Rights only as an acknowledgement that we have them already and were given us by God, and the government agrees to protect them, agreeing that our rights come from a power higher than government.
Follow this legal proof: If our basic rights like free speech and due process and to protect ourself.. come NOT from God, then they don't exist, and can be granted and taken away at will by anyone. We see this happening in the UK recently, where the people are subjects, and their only rights originate from their government, and can be taken away by it. Our rights on the other hand, come from the highest power. Remove that highest power, and there is no legal basis for our rights to even exist.
We are a Christian nation, in that we have built our laws and society around its moral ideals..as much as it is possible. Never before has it been more clear that absence of religion or other religions have other moral teachings that are quite different than say…not killing people..for example. There are whole societies, countries, demographics and legal systems that are other than Christian-based, that do not see it as 'wrong' to kill.