I'm not sure what's more disturbing - the fact that Republican Senator and presidential candidate John McCain thinks the Constitution established the United States as a Christian nation, or that 55% of Americans agree with him.
In case you're wondering, the U.S. Constitution mentions religion in exactly one paragraph - the First Amendment. It says the following:
Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the government for a redress of grievances.
I would like to know how a reasonable person would believe that this text establishes the United States as a Christian nation. Christians will commonly argue in response that the United States was "founded on Christian principles" because the majority of the founding fathers were Christian. That may be true, but if the founding fathers were really that concerned about cementing the place of Christianity in our country's founding, perhaps they would have taken a few minutes to write it into the Constitution. Instead, they made their intentions very clear in the First Amendment - the practice of religion (any religion) would not be interfered with, but no single religion would be endorsed by the government either.
The moral of this story? Read your Constitution folks! Especially you elected officials. The Cornell University Law school has a nice navigable electronic version here.