I was shocked and saddened to open my Washington Post this morning and read about Supreme Court Justice David H. Souter's plan to retire. The high court is about to lose one of its strongest defenders of the church-state wall.

When President George H.W. Bush nominated Souter in July of 1990 to replace William J. Brennan Jr., Americans United immediately began scrambling to gather information on the man. I remember making some phone calls and speaking to Wilbur Glahn, a co-worker and friend of Souter who assured me that the judge was exceedingly bright and not the kind of guy who would come on the court with a personal agenda.

AU remained worried. Souter had been serving on the 1st U.S. Circuit Court of Appeals for just a few months and hadn't ruled on any church-state cases. I was unable to find any public comments by him on that topic. While Bush the elder was never as close to the Religious Right as his son turned out to be, AU was worried that he would appoint an ultra-conservative to kowtow to that movement.

Our concerns only increased after John Sununu, White House chief of staff, assured conservatives that the Souter nomination was a "home run."

Souter did turn out to be a home run – but for our side. Almost immediately he emerged as a strong voice for church-state separation and a leading intellectual voice on the high court for that principle. I always enjoyed reading his opinions in church-state cases.

I can't help but contrast Souter with his fellow justice, Antonin Scalia. Scalia is sarcastic, while Souter is studious. Scalia is bombastic and prone to use hyperbole.   Souter is thoughtful and reserved.

In 1994, for example, Scalia and Souter sparred over Board of Education of Kiryas Joel v. Grumet, a decision by the high court striking down the creation of a special "public" school for a band of Hasidic Jews in New York. Scalia would have allowed the school to continue and wrote a typical acid-laced dissent, ridiculing the idea that giving one religious group its own public school "established" religion.

Souter fired back. "Justice Cardozo once cast the dissenter as 'the gladiator making a last stand against the lions,'" wrote Souter. "Justice Scalia's dissent is certainly the work of a gladiator, but he thrusts at lions of his own imagining."

In 2002, Souter took issue with the court's ruling uphold vouchers for private religious schools in the case Zelman v. Simmons-Harris. "If there were an excuse for giving short shrift to the Establishment Clause, it would probably apply here," Souter wrote. "But there is no excuse. Constitutional limitations are placed on government to preserve constitutional values in hard cases, like these."

The blogosphere is abuzz with speculation about possible replacements for Souter. AU will continue to monitor the situation. We'll work to ensure that another strong defender of church-state separation occupies that slot.

Souter has served his nation admirably for 18 years, and I can understand his desire to retire. But a big part of me wishes he would reconsider.