"When in the Course of human events it becomes necessary for one people to dissolve the political bands which have connected them with another and to assume among the powers of the earth, the separate and equal station to which the Laws of Nature and of Nature's God entitle them, a decent respect to the opinions of mankind requires that they should declare the causes which impel them to the separation."
The founders of the United States declared to all the world, exactly why they were breaking away from English rule and establishing their own independant government. This is what they wrote:
"We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness. That to secure these rights, Governments are instituted among Men, deriving their just powers from the consent of the governed, That whenever any Form of Government becomes destructive of these ends, it is the Right of the People to alter or to abolish it, and to institute new Government, laying its foundation on such principles and organizing its powers in such form, as to them shall seem most likely to effect their Safety and Happiness." (The document goes on to list many specific grievences against the king and his government, as ligitimization for their declaring a break with Brittish rule. Not one of these grievences involves religion in any way.)
Please note that nothing in what these men wrote indicates that they were founding this new nation on or for the sake of Christian ideals. Nothing of what they wrote is a specifically Christian idea. Christ is not mentioned, or even hinted at. The term "God" is included in the text, but only in the most general and universal way.
It is not true that the United States was founded as a Christian nation, or that it's founding was based on Christian principals. The founders discussed this idea at length in correspodance, and they rejected it. A number of the original members of the participants of the nation's founding wanted Christian provisions and declarations included, but when the debate on it was over, they all agree NOT TO DO SO.
It is a LIE being perpetuated by the Christian right that the United States is or was ever intended to be a "Christian Nation", or that it was founded on Christian principals. The United States is not and never was intended to be a Christian nation, and it was never founded on Christian religious ideology of principals. And those who claim otherwise are LYING.