This has nothing at all to do with there being no religious tests allowed. An entirely different matter, and since the liberals tried to bring it up with Romney (whose cult of planet gods and until recently seeing African-Americans as accursed IIRCdoes depart from the foundation of Christianity) it’s important that the GOP voters get the matter straight. Even with no religious tests, all GOP members of Congress are Christian, and devout Christians are overwhelmingly GOP while secular humanists are Dem. There is nothing against a party choosing or not a candidate or nominee for any reason. If you believe God created us and salvation is found only in Jesus, who will you want to choose? Although not the same exactly, it’s like the government can’t prevent people of different religions from marrying, but that doesn’t mean a Christian can’t say they’ll only marry another Christian.
That sounds as if you DO NOT think it's right that there are 5 Roman Catholics on the SCOTUS. Therefore, I pointed out that we can't have religious tests.
Now, you are saying that it would be better if they were from different conservative Christian religions. That seems to indicate that you don't believe in religious tests either. In that we (now) agree.
The real question is why so many of our justices are Roman Catholic. It is not a statistical impossibility, but it is a bit of an oddity.
My personal sense is that at least one of the reasons is that Catholics having better schools.