The first amendment of the U.S. Constitution is seen by many as the greatest part of the Bill of Rights. It gives the people the freedom of speech/religion along with freedom of the press which at the time of passage was rare in the world and in modern times still not available in many countries. It's original text goes as follows "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." Despite being one of the most beloved amendments, it's also seen many abuses in American history.
Originally the first amendment was usually upheld on the federal level but states were tolerated to set up their own churches. Such as Massachusetts which was officially Congregationalist until the 1830s. In the 1947 case Everson v. Board of Education ruled that the Establishment Clause did apply to the states. Justice Hugo Black commented on the ruling as such "The "establishment of religion" clause of the First Amendment means at least this: Neither a state nor the Federal Government can set up a church. Neither can pass laws which aid one religion, aid all religions, or prefer one religion to another ... in the words of Jefferson, the [First Amendment] clause against establishment of religion by law was intended to erect 'a wall of separation between church and State' ... That wall must be kept high and impregnable. We could not approve the slightest breach."