Church-state experts say Supreme Court Justice David Souter has been passionate on religion-related issues, with his own particular blend of views, and that the landscape could seriously shift under a new justice.

Souter is known for voting for and writing for a strict separation of church and state, which for him meant rejecting any taxpayer money for the direct support of religion, including vouchers for religious schools, for example. Interestingly, he also argued strongly that the First Amendment's free exercise clause should be understood to exempt religious practices from laws that burden those practices - even if the government didn't intend to suppress religion.
In other words, he was known for robustly arguing for church-state separation, but also strong support for individual religious freedom.

"He argued that religious practices should receive more protection from government than did his colleague [Supreme Court Justice Antonin] Scalia," said Melissa Rogers, a church-state expert at Wake Forest University who sits on a council that advises President Obama on faith issues. In a 1993 case in which he helped strike down a Florida law aimed at curbing the practice of the Santeria religion (specifically, adherents' practice of animal slaughter as part of worship), Souter wrote that the First Amendment sought to protect believers from having to choose "between God and government."

In a variety of other cases, Souter opposed state aid to religious clubs at public schools and said that state endorsements of religion (such as certain government displays of the Ten Commandments) violated the Establishment Clause. He wrote an elaborate historical analysis in a 1992 case in which the court - along with Souter - said a Rhode Island public school principal violated the law by inviting a rabbi to give a prayer at a graduation and thus endorsing nonsectarian prayers. Checking out a good list of his writings shows strong views that were a mixed bag for some observant groups.