Even if the US is majority Christian, because our government is more secular, the other religions do not have to worry as much. So yeah, when laws are being passed, we cannot appeal to religion and must have a firm ground in secular ideas. If we trusted the Bible for our laws, why not trust the Torah and the Quran and the Analects etc? So to avoid this scramble of religious dogmas, we turn to a secular society in which minority religions are not oppressed
This country was founded on SECULAR values, not christian, not islamic, not buddhist, not hindu. Most of the founding fathers were either Deist (Belief in a higher power, but not a specific God) or atheist. The whole point of leaving England and starting the colonies in the first place was to escape the religious oppression that the church was causing. They didn't want this new great nation to fall into the tyranny of the church of England, or any religion for that matter, and therefore wanted it to be a Government based on Secular ideals.
If you disagree with what I said just go read some books, because you obviously have never studied any history in your life.