Just to play devil’s advocate, for much of the time that the court was all Protestant, the country itself was overwhelmingly Protestant as well.
While that is undoubtably true, IF this was a criteria for appointing Supreme Court justices, Catholics were a sizable enough minority that there should have been at least one Catholic on the court at all times.
All of that however is a moot point since the fact remains that there is NOT, nor should there be, a quota or a religious test for any position in the government. Article 6 of the Constitution guarantees that. A person should be selected solely on their character and their qualifications.