You are correct in saying that the doctrine of church-state separation is not in the Constitution. Hadn't heard about any KKK ties to that doctrine. Do you have a link to an article that will support your statement?
This was used by a KKK (Catholic Hating racist) member from Alabama who was appointed to the Supreme court in a ruling about letting school children ride state or county buses to the Catholic school.
It is known that Justice Black was a KKK member in the 1920's. Keep in mind though that in the South in the 1920's and 1930's, it was not uncommon for lawyers, judges, doctors, etc. to be associated with the Klan.
I'm not defending Black's involvement with the Klan. Though at the same time you cannot just outright label him as a Catholic Hating racist, particularly with references to his time on the Supreme Court.
He wrote or joined in several key opinions during the peak of the Civil Rights era, most notably in Brown v. Board of Education (347 US 483; 1954).
Therefore, I have to advise here that if you do indeed make such an allegation, you really should show us the evidence that supports your claim.
"Separation of Church and State" is the absence of civil authority over religion. Where does the Constitution grant the government any authority over religion or establish a Union between Church and State?
If a Catholic hating racist wrote the opinion then why did the Court rule in favor of the Catholics.