The booklet that came with the recent "Superman on Radio" recordings went into this a bit. The passage is excerpted on this very site, and I've cut and pasted it below:
The Adventures of Superman radio series catapulted into the media spotlight with its "Unity House" story line in 1946. "Recently the Superman program underwent a change as drastic and unprecedented as some of its hero's exploits," wrote columnist Harriet Van Horne, "It became a program with a message." After years of battling mad scientists, atomic weapons and supernatural menaces, Superman took up the battle against racial and religious intolerance when a rabbi and a Catholic priest were menaced by young vigilantes out to destroy an interfaith community house. In the final installment, Superman tells the gang members, "Remember this as long as you live: Whenever you meet up with anyone who is trying to cause trouble between people—anyone who tries to tell you that a man can't be a good citizen because of his religious beliefs—you can be sure that troublemaker is a rotten citzen himself and an inhuman being. Don't ever forget that!"
"Superman is the first children's program to develop a social consciousness," reported Newsweek "To do it, Superman, Inc., the company that controls the Man of Tomorrow in all media, had to sell the idea to the Kellogg. Co. sposoners, and Mutual—two perpetual worriers over the response of reactionary listeners." Officials for both sponsor and network were relieved when the show's plea for tolerance began attracting the highest ratings in the history of the series. "Superman's Hooper rating has risen perceptibly since the change in plot." reported the New York World Tribune on 4 June 1946. "The show is now Number One." The story line attracted endorsements from dozens of organizations including the National Conference on Christians and Jews, the American Newspaper Guild, the American Veteran's Committee, the United Parents Association, the Associated Negro Press and the Boys Clubs of America. After years of protecting his dual identity, Bud Collyer finally stepped into the media spotlight to proudly promote the "Unity House" story line.
Superman also caused a considerable amount of bad feeling among the Ku Klux Klan by mentioning various KKK code words on the program. The code words had been passed on to Superman via the Anti-Defamation League. As a result, Samuel Green, Grand Dragon of the KKK, had to spend part of his afternoon with his ear pressed against the radio. As soon as Superman used a real KKK password, Green sent out urgent orders for a new one. The Grand Dragon is said to have taken this very badly and to have vented his spite on Kellog's Pep by attempting to stop local sales of the cereal. The Kellogg people refused to be intimidated.
The success of the "Unity House" series led to follow-up story lines on juvenile delinquency and school absenteeism. It was the finest moment in the history of radio's greatest adventure program.