Changing Meaning of the First Amendment

Samuel Walker, who is a professor of criminal justice at the University of Nebraska (and is an active American Civil Liberties Union member), wrote the book In Defense of American Liberties, Second Edition: A History of the ACLU. The book is a pro-ACLU reference. Consider the following quote:

"In the 1947 Everson decision, the Supreme Court gave new meaning to the establishment clause of the First Amendment." [underline is mine]

I don't know how much more I need to say about that. The Supreme Court is not supposed to redefine law. Laws are made by Congress, and Congress is the only entity so empowered by the Constitution. The First Amendment was passed by Congress about 150 years before Everson, and one would assume that the authors did not intend for some hidden meaning to be uncovered a century and a half later.