Separation of church and state is a concept that was created during the formation of the constitution, although the term separation of church and state wasn't coined until decades later.

The founding of the United States of America took place in 1787 with the signing of the Constitution, which is a purely secular document. In relation to religion the Constitution states:

Article VI: Clause 3: The Senators and Representatives before mentioned, and the Members of the several State Legislatures, and all executive and judicial Officers, both of the United States and of the several States, shall be bound by Oath or Affirmation, to support this Constitution; but no religious Test shall ever be required as a Qualification to any Office or public Trust under the United States.

In relation to taking the Oath of Office the Constitution simply states:

Article II Section I: Before he enter on the Execution of his Office, he shall take the following Oath or Affirmation:--"I do solemnly swear (or affirm) that I will faithfully execute the Office of President of the United States, and will to the best of my Ability, preserve, protect and defend the Constitution of the United States."

The Bill Of Rights was quickly amended to the Constitution in order to protect the rights of citizens because the original Constitution primarily just defined the powers of government. The third Article of the Bill of Rights (which became the first amendment) states:

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.

The constitution is very clear that religion is to remain out of government!

The founders had every opportunity to create religious mottos, creeds, and pledges, but they didn't do it.  The issue of separation of Church and State is clearly an important one, and one that deserves national debate, but it deserves honest debate with both sides being fairly heard. When an issue is this important we can't afford to simply let rhetoric and ignorance dominate the discussion. The separation of Church and State was central to the founding of the United States of America, more than anything else that was the revolutionary action of the founders that put America at the forefront of the Enlightenment Era and made America a beacon of liberty.

It also needs to be clear to everyone that in trying to preserve secularism in government, or in cases such as the Pledge of Allegiance case, when trying to restore secularism to government, that the secularists are really the ones on the side of tradition. Many who support the use of religious references and religious programs in government claim that the use of religion in government is a matter of American tradition, but in fact it is not. Virtually all of the major references to religion in the federal government have come within the past 50 years. These references do not represent long held American traditions, in fact it can be argued that the mixing of State and religion that has taken place in America over the past 50 years is exactly what the founding fathers warned against.

Besides the danger of a direct mixture of religion and civil government, there is an evil which ought to be guarded against in the indefinite accumulation of property from the capacity of holding it in perpetuity by ecclesiastical corporations.
The establishment of the chaplainship in Congress is a palpable violation of equal rights as well as of Constitutional principles.
The danger of silent accumulations and encroachments by ecclesiastical bodies has not sufficiently engaged attention in the U.S.
- James Madison, being outvoted in the bill to establish the office of Congressional Chaplain

Indeed, the accumulations and encroachments by ecclesiastical bodies have been silently engaged in for some time now, centuries, but the encroachment has been consistent and today it has grown to a level where it is no longer silent.

The separation of Church and State is not meant to hinder religion, or to deny the role of religion in society or in our history. Many of the founders were Christians, even James Madison was a Christian, but what they established was a government that was non-religious - a secular government. That does not mean that they rejected religion; their purpose was to establish a government whose sole function was to administer earthy matters, while matters of religion were left purely to ecclesiastical institutions. The founders, by and large, were very much supportive of the role of religion in society, but they separated the ideas of society and government, leaving religion to play a role in society, not government.