"Wall of Separation"
Introduction
One tactic used by opponents of church/state separation is to discredit the origin of the phrase "wall of separation," as if that would be very relevant to the importance and value of the principle itself. Roger Williams was probably the first to articulate this principle in America, but the idea is forever associated with Thomas Jefferson because of his use of the phrase "wall of separation" in his famous letter to the Danbury Baptist Association. Just how important was that letter, anyway?
Supreme Court decisions through the past two centuries keep referring to Thomas Jefferson's writings as instructive in how to interpret all facets of the Constitution, not merely with regards to First Amendment issues — but those issues do receive particular attention. In the 1879 decision Reynolds v. The U.S., for example, the court observed that Jefferson's writings "may be accepted as an authoritative declaration of the scope and effect of the [First] Amendment."
Jefferson's Wall of Separation in Practice
Jefferson believed in the principle of church/state separation so much that he created political problems for himself. Unlike Presidents Washington, Adams, and all following presidents, Jefferson refused to issue proclamations calling for days of prayer and thanksgiving. It is not, as some charged, because he was an atheist or because he wanted others to abandon religion.
Instead, it was because he recognized that he was only the president of the American people, not their pastor, priest or minister. He realized that he had no authority to lead other citizens in religious services or expressions of religious faith and worship. Why is it, then, that other presidents have assumed that authority over the rest of us?
Madison's Wall of Separation
Some argue that Jefferson's opinion about separating church and state has no relevance because he wasn't around when the Constitution was written. This argument ignores the fact that Jefferson was in constant contact with James Madison, who is largely responsible for the development of the Constitution and the Bill of Rights, and that the two of them had long worked together to create greater religious liberty in Virginia.
Moreover, Madison himself referred more than once to the concept of a wall of separation. In an 1819 letter, he wrote that "the number, the industry and the morality of the priesthood, and the devotion of the people have been manifestly increased by the total separation of the church and state." In an even earlier and undated essay (probably around the early 1800s), Madison wrote, "Strongly guarded...is the separation between religion and government in the Constitution of the United States."
Sources:
Learn Religions online / Other Religions / Atheism and Agnosticism
Jefferson's Letter to the Danbury Baptists
By: Austin Cline; Apr. 5, 2023
https://www.learnreligions.com/jefferson-and-the-danbury-baptists-249666