I'm not sure exactly how Einstein's opinions about god are relevant to the question of whether the U.S. was founded as a Christian nation, but whatever god he may have believed in, it was certainly not the Abrahamic god. He was quite clear that he didn't believe in a personal god, an anthropomorphic god, or a god who intervened in the world or answered prayers. He did use the word "God" from time to time -- so did Khrushchev, as far as that goes -- but the only "god" he believed in was one he described as "[FONT=Arial, Helvetica, Verdana][FONT=Arial,Helvetica,sans-serif]Spinoza's God who reveals himself in the orderly harmony of what exists, not in a God who concerns himself with fates and actions of human being[/FONT][/FONT]." He said that it would be accurate to call him an agnostic, and that from the point of view of a Jesuit he was an atheist. Certainly, whatever god-concept he may have believed in, it was alien to the god-concept of Christians who trot out his Spinoza quote in support of their faith.
To return to the thread topic, most of the founding fathers were nominal Christians, but most of them would not have been accepted as Christians by our modern Evangelicals, and whatever their personal beliefs might have been, they made it absolutely clear that they weren't founding a "Christian nation."