It has pleased the Providence of the first Cause, the Universal Cause, that Abraham should give religion not only to Hebrews but to Christians and Mahomitans, the greatest part of the modern civilized world.
-- John Adams to M.M. Noah, July 31, 1818.
Interesting news that John McCain rejected the endorsement of and was consequently unendorsed by Rod Parsley and John Hagee. This illustrates the tension between orthodox Christianity and Americanism. I know that Parsley and Hagee are sort of goofballs and probably do not well represent orthodox Christianity. However, Parsley's notion that Islam is a false religion is an authentic teaching of orthodox Christianity, just as Hagee's idea that Roman Catholicism is an apostate religion is an authentic teaching of orthodox evangelical Protestantism.
Orthodox Christians need to grapple with the question “can one be a good Christian and a good American at the same time?” The two don’t always mix well and Christians would be well warned to distinguish between them. “Americanism” as put forth by America’s key Founders holds that most or all religions (including ISLAM -- see Adams' above quotation) are valid "religions."
John McCain, as potential President, is going to be encouraged to tow the line of Americanism — as such he’s going to have to welcome Muslims as equal citizens and their faith as valid. The trouble he’s having with Hagee & Parsley well illustrates the tension between orthodox Christianity and Americanism. That Islam or arguably any non-Christian faith is “false” is a profoundly un-American idea and will not mix well in Presidential politics.
John McCain's faith -- that he believes in God and is a "Christian" in probably a nominal sense -- well positions him for the American Presidency. The early Presidents and Abe Lincoln were NOT orthodox Trinitarian Christians and as such they helped establish a tradition of making anyone who believes in God feel fully included as an "American" citizen. America went from being almost all Protestant to religiously diverse because of the principles put forth by Washington, Adams, Jefferson, Madison, and Lincoln, not in spite of them.
There have been orthodox Trinitarian Christians as Presidents like Jimmy Carter and the current President. But they still tend to follow the tradition established by the early non-orthodox Presidents of pretending all religions worship the same God. Presidents, as public figure heads, are encouraged to talk about God -- quite a bit actually -- but downplay or avoid specific doctrines like Trinitarian orthodoxy.
You could tell Ronald Reagen devoutly believed in God from his public proclamations; but I'm hard pressed to find evidence of his traditional Christianity from the public speeches he made as President (I'm not even sure if he was one).
When George W. Bush claimed the Muslims' "Allah" was the same God he, as a Christian, worshipped, he was being a "good American" President, not necessarily a good orthodox Trinitarian Christian.