Which Article and Section or which Amendment of the US Constitution mandates the separation of...?

Good question. Although those actual words do not appear, the First Amendment reads, "congress shall make no laws respecting an establishment of religion, or prohibiting the free exercise thereof."

I think that's pretty clear, but Thomas Jefferson later clarified this in a letter to the Danbury Baptist Association, dated January 1, 1802:

"Believing with you that religion is a matter which lies solely between man and his God, that he owes account to none other for his faith or his worship, that the legislative powers of government reach actions only, and not opinions, I contemplate with sovereign reverence that act of the whole American people which declared that their legislature should 'make no law respecting an establishment of religion, or prohibiting the free exercise thereof,' thus BUILDING A WALL OF SEPARATION BETWEEN CHURCH AND STATE."

Contrary to what many would have you believe, the Founding Fathers did NOT intend for the United States to be a Christian nation. Quite the opposite in fact, since freedom of religion is arguably the MOST important element of any free and democratic society. And religious freedom cannot exist without a strong separation of church and state.

Most of the Founding Fathers believed in God, but many (including Ben Franklin, Thomas Paine, George Washington, and Thomas Jefferson) were deists - they had serious reservations about organized religion in general, and specifically Christianity. Paine once wrote, "of all the systems of religion that ever were invented, there is none more derogatory to the Almighty, more unedifying to man, more repugnant to reason, and more contradictory in itself, than this thing called Christianity."

Many believe that the U.S. was founded on Christian principles, but this was simply not the case. John Adams addressed this assertion directly in the Treaty of Tripoli (1796): "The government of the United States of America is not in any sense founded on the Christian Religion."

As Thomas Jefferson put it, “I am for freedom of religion, & against all maneuvres to bring about a legal ascendancy of one sect over another.” This is precisely why Christian-themed icons & groups need to stay out of government institutions – because their inclusion marginalizes the non-Christian citizens of a country in which “all men are created equal.”

The freedom to worship (or not) as we choose is one of the things that makes the U.S. a GREAT place to live.

1st Amendment says: 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.