A letter from Marcia Swift of Humanists of Linn County regarding commitment to separation of church and state argued that the First Amendment and judicial interpretations make it clear that government cannot endorse any particular religion.
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 peaceable to assemble, and to petition the Government for a redress of grievances.”
The term “separation of church and state” is not found in the Constitution. It is from a 1947 court case which took the term out of context from some of President Thomas Jefferson’s letters exchanged with the Baptist Association of Danbury, Connecticut, after he became president. An excellent discussion can be found at: https://wallbuilders.com/separation-church-state/. Another court case of note was in 2014, American Humanists vs. United States, whereby a federal inmate sued because he couldn’t form a Humanist group in prison even though groups for Christians, Muslims, etc. were allowed. The Humanists argued that Humanism should be treated like the religions, and indeed “The court finds that Secular Humanism is a religion for Establishment Clause purposes.”
The argument from the Humanist organization for separation of church and state seems ludicrous based on their own status as a religion and plain old common sense.