WASHINGTON – The Supreme Court agreed yesterday to consider, for the first time in a quarter century, the constitutionality of public displays of the Ten Commandments, opening the door to a ruling that could redefine the legal boundaries between religion and government.

With courts across the country reaching conflicting conclusions about when and where the Decalogue can be displayed without a violation of the separation of church and state, the top court has been asked to set out a new and clear rule.

The court has agreed to hear in February two cases in which courts have reached opposite conclusions. The U.S. Court of Appeals for the Fifth Circuit upheld the display of a 6-foot-tall granite monument etched with the Commandments on the grounds of the Texas state legislature in Austin. In a Kentucky case, the Sixth Circuit ordered the dismantlement of a display of the Commandments in courthouses.

“This will be the blockbuster religious liberty case of the term. It’s going to affect every Ten Commandments case in the country, and the interpretation of the First Amendment state church provision in the Constitution,” the president and general counsel of the Liberty Counsel, Mathew Staver, said. That group is representing Kentucky’s McCreary County.

The court’s pronouncements next year could affect references to God in such official contexts as the Pledge of Allegiance and the national motto, Mr. Staver said.

Last year, the high court considered whether recitations in public classrooms of the words “under God” in the pledge violated the First Amendment, but the judges dismissed the case on a technicality without reaching the constitutional issue.

The Supreme Court last dealt with the Ten Commandments in 1980, when it struck down a Kentucky law that required their posting in public schoolrooms. The case was decided without oral argument or written briefs.

Federal courts have been applying with contradictory results a rule set out by the Supreme Court in a 1971 case, Lemon v. Kurtzman. It states that government action must have a secular legislative purpose, cannot have the effect of inhibiting or advancing religion, and must not foster “an excessive government entanglement with religion.”

Currently, four federal circuit courts and one state Supreme Court hold that displays of the Ten Commandments are constitutional, while three federal circuit courts hold that such displays are unconstitutional, according to Mr. Staver. He is urging the court to craft a new, clearer rule that would be more hospitable to public references to God or the Ten Commandments.

“Simply put, there is a big different between acknowledgment of religion and the establishment of religion,” he said. “A display of the Ten Commandments does not establish religion.”

In both the Kentucky case and the Texas case, the Ten Commandments were displayed among other historical monuments and artifacts, and the governments argued that they were not meant to endorse a religion.

In the Texas case, a homeless man with a law degree, Thomas Van Orden, sued the Texas governor, Richard Perry, to remove a monument he said promoted the commandments as a personal code of conduct for youths, and favored the Christian and Jewish faiths.

The lower courts declined to order the display removed, noting that it was one of 17 historical monuments on the grounds, including a tribute to African-American legislators, a Confederate plaque, a display on the floor of the Capitol Rotunda featuring the Mexican eagle and serpent symbolizing Aztec prophecy, and the Confederate seal containing the inscription “Deo Vindice” (God will judge). The placement of the monument is “appropriate because it is a monument to a historical document that has been a founding principle of this nation,” said a spokeswoman for Governor Perry, Kathy Walt.

In the Kentucky case, the American Civil Liberties Union of Kentucky is arguing that postings of the Ten Commandments in courthouses in Mc-Creary and Pulaski counties “conveyed a message of religious endorsement.”

The Sixth Circuit agreed last March that the displays violated the First Amendment, though they were surrounded by other historical documents, such as the Declaration of Independence, the national motto, and the text of the Mayflower Compact.

“The Ten Commandments advocate believing in God, observing the Sabbath, and not worshiping idols. Those are religious beliefs that should be left to each individual,” said the general counsel for the ACLU of Kentucky, David Friedman, who argued the case in the appeals court.

“Especially in a courthouse, people should not be made to feel like outsiders in their own community because they may not share the prevailing religious view,” he said in a statement.

The court’s decision to hear the cases comes shortly after the judges declined to hear an appeal by the former chief justice of the Alabama Supreme Court, Roy Moore. He was challenging a decision to oust him from his job for defying a court order to remove a granite monument of the Ten Commandments from his courthouse.

Mr. Moore said he was not surprised that the Supreme Court accepted the cases while denying his, because he had refused to portray the Ten Commandments as historical rather than religious artifacts.

“It’s discouraging that the two [cases] that are up there have surrounded the monuments and plaques with historical documents and artifacts, only to secularize the displays to become acceptable to the federal courts,” he told The New York Sun in a telephone interview.

“Both of these cases proceed on the principle that the state can only acknowledge God if it first denies his sovereignty. My case was different, and that’s the reason they locked the Ten Commandments in the closet,” he said.

The president of the American Humanist Association, Mel Lipman, said the top court’s treatment of Mr. Moore suggests the judges may be unwilling to endorse the public display of the Ten Commandments.

“We’re hopeful that this announcement indicates the court’s willingness to make the obvious clear for the lower courts: Government displays of the Ten Commandments are unconstitutional, whether they are new or old, and whether they stand alone or next to secular displays,” he said in a statement.