The American Civil Liberties Union sued the city of Slidell on Tuesday for displaying a painting of Jesus in a courthouse lobby, saying it violates the constitutional separation of church and state.

The ACLU sued after the Slidell City Court refused to voluntarily remove the picture and a plaque below it that reads: "To Know Peace, Obey These Laws." The ACLU says the portrait — an image of Jesus presenting the New Testament — is a religious icon of the Eastern Orthodox branch of Christianity.

"We did not file this lawsuit because the ACLU is anti-religion ... We did file this lawsuit because we believe this display is clearly in violation of the law," said Vincent Booth, president and acting executive director of the Louisiana ACLU chapter.

The suit was filed on behalf of an unidentified person who complained to the ACLU about the picture. Named as defendants were the city of Slidell, St. Tammany Parish and City Judge James Lamz. St. Tammany Parish is being sued because it partially funds the court, the ACLU said.

On Saturday, Lamz said the picture would stay up unless a federal judge ordered it removed. He said he didn't believe the portrait violates the Constitution, but the issue should be decided in federal court.

Lamz could not comment Tuesday because of the pending litigation, his office said.

Before refusing to take the painting down, Lamz consulted Douglas Laycock, a professor at the University of Michigan Law School who has argued before the Supreme Court.

Laycock said he told Lamz that the legal issues in the case aren't clear-cut and could set legal precedent.

"I don't know how far the two sides will want to push things," Laycock added.

The painting has been on display at the courthouse for nearly a decade and hadn't provoked any complaints prior to the ACLU's recent objections, said Michael Johnson, senior legal counsel for the Alliance Defense Fund, a Christian civil rights group representing the city and parish.

Johnson, whose group is often at odds with the ACLU, said the painting sends an inclusive message of equal justice under the law. He said the U.S. Supreme Court has ruled that similar displays in public forums are constitutional.