Since I’ve been on the road talking about Justice Scalia, I’ve consistently been asked about the fact that six of the nine justices are Roman Catholic and about how that affects the Court’s opinions, particularly those of Scalia. I devoted a chapter to Scalia and religion — Passions of His Mind — and, despite how touchy the intersection of personal belief and judicial views can be, I am ready to field these questions.

Recently, however, I’ve felt a shift in the line of questioning, as happened this week at Yale, and it comes down to a query fraught with more hazards: Would it be politically difficult for the president to choose a Catholic for any new vacancy? And would it be almost as difficult to pick someone who is Jewish? Right now, serving with the six Catholics (Chief Justice John Roberts and Justices Antonin Scalia, Anthony Kennedy, Clarence Thomas, Samuel Alito, Sonia Sotomayor), are two Jewish justices (Ruth Bader Ginsburg and Stephen Breyer) and one Protestant. That one Protestant, Justice John Paul Stevens, will turn 90 in April and is likely to be the next to retire. I’ve heard a few professors say if Stevens steps down, it would be nearly impossible for Obama to chose a Catholic, or maybe even someone who is Jewish, because of the current court makeup.

I do not know how religion would affect the choice, but here’s some context from where I sit, along with the thoughts of two of my subjects:

The Supreme Court had long been dominated by Protestants, of course, like the White House and Congress. When Catholics became a five-member majority after the 2005 and 2006 appointments of Roberts and Alito, it barely made the news, however, because of faded anti-Catholic sentiment and evolving religious pluralism in America. After recent abortion-rights disputes, the religious backgrounds of the justices became a topic for discussion, but still with unease. As I’ve written, lawyers often express thoughts about how Catholicism influences legal opinions only privately, reflecting their discomfort with the suggestion that either Scalia is being disingenuous about how his beliefs influence him (he says they do not) or that serious Catholics cannot think nonreligiously about legal matters.

For his part, Justice Scalia has said he was “very pleased and sort of proud that Americans didn’t pay any attention” when the bench suddenly had a Catholic majority.

My earlier biographical subject, Sandra Day O’Connor, an Episcopalian, comes at it differently. And when I was asked about the justices and religion in New Haven this week, I found myself recalling something she said last fall. During a panel discussion, O’Connor emphasized the value of diversity on the bench — in sex, professional experience, geography and religion. “I don’t think we should have nine clones up there,” O’Connor said, primarily complaining at that point that all current justices had been elevated from lower U.S. appeals courts. When asked about geographical representation, O’Connor took the initiative to go beyond the question and declared, “I don’t think they should all be of one faith and I don’t think they should all be from one state.”

As we get ready to enter 2010, Supreme Court watchers – living in a tight and sometimes unhelpfully focused world (see 2005, below) — already have been considering who might be next for the Court. In the last two weeks, I have heard the question asked at holiday parties, during dinners and in private conversations. The most likely scenario is that Justice John Paul Stevens, who turns 90 in April, will announce his retirement in the spring, and that the Obama administration will work with a list that begins with the also-rans from spring 2009 and is expanded with other women and, likely this time, a few men. When Sonia Sotomayor was chosen last May to succeed David Souter, President Obama was determined to add a second woman justice to the nine-member bench and did not interview any male candidates. Some of the very premature discussion among Court insiders these days has centered on the current chances of candidates from the 2009 short-list: Would Homeland Security Secretary Janet Napolitano be a less likely choice because of fallout from the Christmas Day bombing plot involving a Nigerian student allegedly linked to al-Qaeda and her comment that “the system worked”? Have Solicitor General Elena Kagan’s arguments before the Court helped her chances? Would Appeals Court Judge Diane Wood, who turns 60 next July, be less likely now because of her age? Generally, I should note, President Obama has not been seeking younger candidates for the federal bench as GOP predecessors Ronald Reagan and George W. Bush did.

 I have found that all this preliminary speculation has taken a new twist, suddenly involving whether Justice Stevens would be the next to step down. I have long thought he would. When I interviewed him in October, he talked readily about his legacy and that he had not hired a full staff for the next term. When I asked him about media attention on the fact that he had not selected the usual contingent of law clerks, he said, “That can’t be news. I’m not exactly a kid.” While the justice would not confirm his retirement plans, it seemed to me his bags were all but packed. In recent days, however, a few Court watchers have been raising doubts about whether liberal Stevens would really be the next to go and whether a more likely possibility could be the retirement of one of the Court’s older conservatives. That would be far more consequential. If Obama were to replace a conservative justice on this closely divided bench, it would tilt the Court in a new direction and have a greater effect on the law of the land.

 As far as I can tell, none of this very mildly percolating speculation about a conservative leaving is rooted in reality, and I would throw water on all of it — if 2005 did not remain so vivid in my mind. That was when, as we all were watching for the retirement word from Chief Justice William Rehnquist, ill with thyroid cancer, Justice Sandra Day O’Connor announced her exit. I remember too well that July 1 day when she went public. I had been finishing up my biography of her and in denial about the effect of her husband’s Alzheimer’s disease on her plans. I had known for years of his illness, yet I had seen her conquer so much over the course of her pioneering life that I believed his situation would not lead her to leave. I was wrong, as were many of my veteran court colleagues (“Most in the Media were Blindsided by Justice” read a Washington Post Style-section headline). Several of O’Connor’s fellow justices were shocked, too. Justice Scalia told me later that July he had heard the news on the car radio as he was coming into work and nearly driven off the road.

 All this was in the back of my mind last night when a close friend and an astute Court observer asked what I made of some talk that a conservative justice, maybe even Scalia, might retire. The facts cut both ways: Justices Scalia and Anthony Kennedy, the eldest on the conservative side, will each turn 74 in the new year. That’s young in justice-years, although not for the usual worker. Because of generous judicial pensions, both men could retire and earn their salaries for life. Justice Stevens told me, “You think about retiring when you get to the point that you’re working for free” – even though that clearly did not make the difference for him in his 60s, 70s, or even 80s. Scalia and Kennedy have had some minor health problems through the years but appear in good enough physical condition. Scalia, who smokes, eats and drinks in great quantities, comes from long-living kin. Finally, I think about whether either would want to end his legacy now and give Democratic President Obama an opportunity to name a successor? I just don’t see it – especially in the case of Scalia.  So I told my friend that I’d still put my money on Stevens to leave first. Yet, as I cannot dismiss the events of 2005, I must add that if I suddenly heard Scalia was leaving, I wouldn’t drive off the road.