I'm looking in vain through the Constitution where it requires a religious balance of any kind. All I can find is some nonsense in Article 6 about "no religious Test shall ever be required as a Qualification to any Office or public Trust under the United States."
It doesn't seem to have been an issue in the times of our history when the SCOTUS was all Protestant.
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.