Today, September 17, is the day that the United States celebrates Constitution Day. It was on this day in 1787 that the delegates at what is now known as the Constitutional Convention signed the document that is now the supreme charter of the US government. Some people mistakenly think that the Constitution is a Christian document. While some of the founding fathers were no doubt Christians, as I've mentioned previously on this blog, the Constitution itself has very little to say about religion in general, and nothing about Christianity specifically (although I would argue that it is reasonable to view the discussions of religion in the Constitution with a reference to Christianity).
There is one notable point in the original Constitution (sans amendments) that deals with religion. Article VI, Section 3 states that "no religious Test shall ever be required as a Qualification to any Office or public Trust under the United States." This statement indicates that an atheist has as much right to serve the US as a God-fearing evangelical does. While there have been many people in US history that have attempted to dissuade the government from employing Catholics, Jews, and people of other faiths (most recently Muslim) as government officials, the Constitution prohibits such discrimination on religious grounds. This is an important point to understand. The rest of the Constitution goes about the rather contentious task of actually setting up a government.
Freedom of religion was not even a part of the Constitution as originally submitted to the states. I would argue that many on both the religious and secular sides of the debate misunderstand this amendment. Some would argue that the United States has been a Christian nation. Most of the time, the idea of a Christian nation is tied to the "Christendom" of medieval Europe. State-supported religion is not exactly a great spur to individual piety in most instances. Individual piety is just that, individual. While the US had a large number of professed Christians in its founding era, it was not a "Christian nation." Secularists argue that there should be no interaction between religion and public life, arguing for the "wall of separation." This wall is not specifically mentioned in the Constitution, either. It is very difficult to have people who do not have beliefs that influence and even motivate their behavior. Secularism is inherently a belief system, as is Christianity. It is a bit hypocritical on both sides, in my humble opinion for either to say people can have their beliefs, but they cannot let them influence their public personna. (Note that the argument is not limited to Christians and secularists, but could involve any number of other groups.)
The first amendment has several important, closely tied freedoms established. First, there is freedom from an established state church, while there is maintenance of the right for individual freedom to worship in any manner. There are well-known instances of public officials (i.e., government officials) in early American history holding prayer in their public capacity. Therefore, it cannot reasonably be argued that the founders wanted freedom from religion. The freedom of speech is closely tied to the freedom of religion, although political speech is most clearly in view here. The right of peaceable assembly is also important for religious groups, whether they be Christian or not. How could church meetings be protected without this?
The Constitution is not a religious document. More specifically, it is not a Christian document. However, it does guarantee freedom of religion for all. This is very important. Even if it specified Christianity, which branch would be favored? That would necessarily bring up all sorts of problematic issues.