During the first 47 years of the existence of the U.S. Supreme Court, all of the justices were Protestant. The first Roman Catholic on the Court was Chief Justice Roger Taney, the author of the infamous Dred Scott decision, which held that African Americans were not citizens with standing to sue in federal court, and which contributed to the outbreak of the Civil War.
Although their monopoly had been broken, Protestants continued to dominate Supreme Court appointments for many decades.
In 1916, President Woodrow Wilson nominated Louis Brandeis as the first Jewish justice.
Another milestone was passed in 1994, when Stephen Breyer, who is Jewish, was nominated by President Bill Clinton. For the first time, Protestants were in the minority; there were four of them, serving with three Roman Catholic justices, and two Jewish justices.
Now, if Elena Kagan, who is Jewish, is confirmed to replace John Paul Stevens, there will for the first time be no Protestants on the Court. Chief Justice Roberts, and Justices Scalia, Kennedy, Thomas, Alito and Sotomayor are Roman Catholic, and Justices Ginsburg and Breyer, and Justice-designate Kagan are Jewish.
None of this is to say either that 1) I think adherents of any religion are inherently more qualified for any government office, including Supreme Court justice, than adherents of any other religion; or 2) I feel oppressed as a white, male, Protestant (my being gay is another matter).
I just find it interesting that there has been such a quick and thorough turnaround from such a strong preference for Protestant justices. Much has been said about the predominance of judges from the northeast, with Ivy League credentials, on the current Court. But those numbers make clear that this Eastern Establishment is not the same Eastern Establishment that existed before about 1980.