Where did the modern understanding of "separation of church and state" come from? If you said Thomas Jefferson, you're wrong. It was former U.S. Supreme Court Justice Hugo Black, whose anti-Catholicism highly influenced his 1947 ruling in Everson v. Board of Education that the First Amendment imposes a "high and impregnable" wall between church and state. For more information on this, see Separation of Church and State by University of Chicago law professor Philip Hamburger and Thomas Jefferson and the Wall of Separation Between Church and State (Critical America) by American University professor Daniel Dreisbach.