Separation Of Church And State: The ‘So-Called’ Principle That Has Been Protecting Our Rights Since 1791 | Americans United for Separation of Church and State

…Consider, for example, Thomas Jefferson, with whom the phrase "separation of church and state" is strongly associated. 

Jefferson invoked it in an 1802 letter to a Baptist group in Danbury, Conn. The Baptists knew Jefferson was a champion of religious freedom and wrote to him to express their discontent with the situation in Connecticut, where a quasi-established church run by Congregationalists ruled. In his reply, Jefferson spoke of the First Amendment creating "a wall of separation between church and state."…

https://www.au.org/blogs/Gorsuch-so-called