The founding fathers knew a little something about the government support of religion. In fact, they spent a lot of time rebelling against an empire that ran its own church. When writing the Constitution of the United States, they knew that the religious upheaval and conflict caused by state-sponsored religion was not something that the new country could afford. What had to be done was to separate the two, and create a government that did not interfere with nor support religion. They did this in two ways in our Constitution, but I'm just focusing on this:
"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."
Before I talk about this, you have to know that the Constitution does not grant individuals rights: it merely affirms that the government cannot conflict with them. In this case, the framers worded it in a way that meant that the government could not interfere with or support any religion. This does not only prevent them from establishing a state religion (thus, the idea of a "Christian nation" is already out of the window legally), but it prevents them from doing things on the small scale regarding religion. For example, they cannot teach creationism in public schools, mandate prayer, have the ten commandments on display in a government building, and so on and so forth. Those beliefs and practices are best left to the privacy of one's home, and the framers knew that. In essence, by recognizing that the government cannot prohibit private practice of religion, they have created a public protection from it. It becomes such a case where the government cannot impose religion on its subjects, because doing so would violate the free practice of religion. If you have a government that professes a specific religion or belief system, let's say Christianity, then the government has undermined your right to believe something else. Even if they allow all other beliefs and religions, you are still in a place where the government recognizes one belief over another (most importantly, yours). Since it is impossible for the government to support every religion and belief system with the intent of treating them all fairly, the only other option to consider is to stay away from all of them and remain secular.
While the first amendment does not specifically say that there is a separation from church and state, it was never intended to. That separation is something that arises from this amendment and the prohibition of religious tests for office. It is a concept born out of a simple rule: that the government cannot support religion.
There are things being done by government today that clearly violate this principle. Firstly, the fact that we have "In God We Trust" on our money. This is clearly an example of state-sponsored religion, and is misleading because it implies that the entire country accepts that statement to be true (of course, it's not, which is why the government shouldn't play with religion). Secondly, the phrase "under God" in the Pledge of Allegiance. This is another example of the state sponsoring a religious belief. It also becomes apparent that the state is sponsoring a specific belief (Christianity) when you look at the history surrounding the legislation that added that phrase.
Just a side note, the same rule applies for opposite religious or theistic beliefs. The government does not have a right to claim that there are no gods. It must work from a stance of indifference.
So, to conclude, remember this: you are free from government sponsored religion. Just because the majority happens to hold a certain belief does not render them more powerful. The government does not recognize (or rather, should not recognize) a majority or minority. In instances where those things do arise, it's best to just stay out of it.