Where is Separation of Church and State found in the US Constitution and how does the Supreme Court interpret this clause?

Mandate for the separation of church and state is in the US Constitution.

It is implied in the Constitution's First Amendment "Establishment Clause," where "Congress shall make no law respecting an establishment of religion..." has been widely interpreted to mean that the government should not involve itself in religion in any way, including providing funds to support religious teaching in public schools, or allowing organized prayer in activities or facilities that receive government funds.

The First Amendment's freedom of religion includes the freedom to refrain from practicing a religion or having someone else's religious rituals inflicted upon a person in a public (non-religious) setting.

Cornell University Annotations on the First Amendment contrasts Justice Joseph Story's 19th-century interpretation of the Establishment Clause, which discusses the attitude toward Christianity, with Justice David Souter's 20th-century interpretation, which takes into account the variety of religious practices (or lack thereof) in the United States today.

Thomas Jefferson firmly believed the Constitution should erect a "wall of separation" between church and state, as he explained in his 1802 letter to the Danbury (Connecticut) Baptists: "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 legitimate 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.

Adhering to this expression of the supreme will of the nation in behalf of the rights of conscience, I shall see with sincere satisfaction the progress of those sentiments which tend to restore to man all his natural rights, convinced he has no natural right in opposition to his social duties."

In the case Reynolds v. United States, 98 US 145 (1878), Chief Justice Waite delivered the opinion of the Court, quoting Jefferson's in the context of a Mormon polygamy trial, and concluding: "Coming as this does from an acknowledged leader of the advocates of the measure, it may be accepted almost as an authoritative declaration of the scope and effect of the amendment thus secured.

Congress was deprived of all legislative power over mere opinion, but was left free to reach actions which were in violation of social duties or subversive of good order.

"Justice Story "Probably, at the time of the adoption of the constitution and of the amendment to it, now under consideration, the general, if not the universal, sentiment in America was, that Christianity ought to receive encouragement from the state, so far as was not incompatible with the private rights of conscience, and the freedom of religious worship.

An attempt to level all religions, and to make it a matter of state policy to hold all in utter indifference, would have created universal disapprobation, if not universal indignation."

[Cornell Commentary: "The object, then, of the religion clauses in this view was not to prevent general governmental encouragement of religion, of Christianity, but to prevent religious persecution and to prevent a national establishment."]

Justice Souter "[Justice Story's] interpretation has long since been abandoned by the Court, beginning, at least, with Everson v. Board of Education, in which the Court, without dissent on this point, declared that the Establishment Clause forbids not only practices that "aid one religion" or "prefer one religion over another," but as well those that "aid all religions.""

[Cornell commentary: "Recently, in reliance on published scholarly research and original sources, Court dissenters have recurred to the argument that what the religion clauses, principally the Establishment Clause, prevent is "preferential" governmental promotion of some religions, allowing general governmental promotion of all religion in general.

The Court has not responded, though Justice Souter in a major concurring opinion did undertake to rebut the argument and to restate the Everson position."]

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Amendment I "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."