Why is there separation of church and state in the United States?

The "separation of church and state" is derived from two statements in the First Amendment of the US Constitution: the "establishment clause" and the "free exercise clause" that immediately follows, which are: "Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof..."

Thomas Jefferson's opinion on this matter was influenced by the English philosopher John Locke, who argued that the government lacked the authority in the realm of individual conscience. Jefferson wrote that religion is between a man and his God, and was among the first to describe the First Amendment as a "wall of separation" between Church and State. However, this did not mean that Jefferson believed that the State should be entirely divorced from religion; as President, Jefferson (for example) used Federal funds to support Catholic ministries among the Indians. James Madison wrote that he would have preferred that the First Amendment read "no religious doctrine shall be established by law" and stated that his interpretation of the First Amendment was that Congress should not establish a religion and enforce the legal observation of it by law. Both Jefferson and Madison supported the Virginia Statute for Religious Freedom, which states that no man can be compelled to participate in any religious activities.

In the subsequent two centuries, the US Supreme Court has referenced the separation of Church and State on many occasions. The Court has ruled that certain rules of society cannot be violated under this clause; for instance, the Mormon practice of polygamy was ruled illegal and not a right under religious freedom. Cases involving the use of otherwise-illegal hallucinogenic drugs as part of religious ceremony have gone both ways. Justice Scalia has commented that the separation of Church and State has been used as a "bulldozer" to remove all references to religion from public life. This has not deterred Presidents from making open reference to their religious faith while in office. It is generally acknowledged that in the US people have more freedom to worship as they please (or not) than in most other parts of the world.