One of the defining features of the constitution of the United States is that it is founded on the principle of the separation of religion and state. From James Madison's victory during the "Virginia Assessment Controversy" to Thomas Jefferson's vision of a "wall" of separation to the First Amendment, our commitment to religious liberty has been two-fold. First, the government cannot favor or "establish" any religion over others. Second, the government cannot impede a person's ability to freely "exercise" his/her faith (to the extent that it not harm others). The purpose of this separation was to protect the integrity of both government and religion.
Despite the constitutional guarantee, the U.S. has not always lived up to the ideal of the separation of religion and state. Early laws favored Protestant Christians over Catholics, Jews and First Nation peoples. However, guided by the First Amendment, the general trend has been an increase in religious liberty. Since the beginning, there have also always been people who oppose this principle, who have sought to favor their own belief systems - for example, the so-called "Faith-based initiatives" and attempts to define marriage as "between a man and a woman." Thus, the preservation of the wall of separation between religion and state requires constant vigilance.