I finally got around to cataloging a lot of covers that I acquired a few months ago and came across this label on a cover postmarked in Monrovia, CA on August 16, 1959.

I initially thought that the label may have been associated with the run-up to the 1960 U.S. Presidential election in which John F. Kennedy would defeat then-Vice President Richard Nixon and become America's first Catholic President.
But now I'm wondering if it was something used to advance the cause of the group Americans United for Separation of Church and State which had been formed in 1947 and opposed any encroachment on the first clause of the First Amendment to the U.S. Constitution.
If anyone has information on the origing of this label I'd appreciate hearing from you.