Respectfully, most religions have similar beliefs regarding kindness toward others, and many atheists are kind to others too.

As far as the founding fathers go, they were not all Christians, but even if they were, they still made sure to include freedom of religion in the Constitution, honoring the basic human right to choose a belief system. The country was founded on religious freedom, not Christianity.

Lastly, there are a whole lot of tax paying Americans who are not Christians. To declare the country as a Christian country would be disrespectful and unfair to them. There are many reasons religion is losing ground, and I think most of those reasons have to do with non-humanitarian beliefs and practices.