McCain: "the Constitution established the United States of America as a Christian nation."
Number of times the Constitution actually mentions Christianity: 0.
Number of times the Constitution forbids the establishment of religion by Congress: 1.
Undoubtedly, given the demographics of the time, the Framers expected that the American population would be primarily Christian, and some of them believed that the ideas that inspired the Declaration and Constitution were Christian in origin (though that cuts both ways, because southern slavery advocates believed that slavery, protected to some extent by the original Constitution, was divinely ordained). But that's not quite the same as suggesting that the American constitution establishes a "Christian nation."