President Trump signed an executive order Friday instituting “extreme vetting” of refugees, aimed at keeping out “radical Islamic terrorists.”

“I’m establishing a new vetting measure to keep radical Islamic terrorists out of the United States of America,” Trump said during his signing of the order. “We don’t want them here. We want to make sure we are not admitting into our country the very threats our soldiers are fighting overseas.”

According to drafts of the executive action, the order bars people from the Muslim-majority countries of Iraq, Syria, Iran, Sudan, Libya, Somalia or Yemen from entering the United States for 30 days and suspends the U.S. Refugee Admissions Program for 120 days. The program will be reinstated “only for nationals of countries for whom” members are vetted by Trump’s administration.

In an interview Friday with the Christian Broadcast Network, Trump said he plans to help persecuted Christians.

“Do you know if you were a Christian in Syria it was impossible, at least very tough, to get into the United States?” Trump said. “If you were a Muslim you could come in, but if you were a Christian, it was almost impossible and the reason that was so unfair, everybody was persecuted in all fairness, but they were chopping off the heads of everybody but more so the Christians. And I thought it was very, very unfair.”

In a statement, the American Civil Liberties Union declared Trump’s action “just a euphemism for discrimination against Muslims.”

From both legal and historical perspectives, the plan to ban refugees from specific countries is within the powers granted to the president under current law and historical precedent, according to Charles Haynes, vice president of the Newseum Institute’s Religious Freedom Center. However, whether the president can limit the ban to one religious group is another question.

Many Muslims, especially Shiites, are among the religious minorities under attack, Haynes said. This “raises moral and humanitarian concerns about excluding them from entrance to the U.S. while permitting people of other faiths,” he said. “Whether this policy rises to the level of a constitutional violation is uncertain and will be debated by constitutional scholars in the coming weeks.”

Issues related to the Constitution and religion are usually associated with matters of sex, such as contraceptives and LGBT discrimination, but some observers said they expect Trump’s actions on immigration to raise new challenges for religious freedom, according to Chelsea Langston Bombino of the Institutional Religious Freedom Alliance at the Center for Public Justice. Several organizations, she noted, are speaking out against orders that “will hurt the very people that their organizations were established, out of a religious calling, to serve,” she said.

Trump’s actions have been decried by several religious groups this week. “The expected cutbacks to U.S. refugee programs and funding will compromise our ability to do this work and the infrastructure needed to serve refugees in the years to come,” evangelical ministry World Relief said in a statement.

And in a strongly worded statement, Rabbi Jack Moline, the Interfaith Alliance president, noted that this decision was announced on International Holocaust Remembrance Day.

“For decades, the United States has prided itself as a safe bastion for refugees around the globe escaping war and persecution,” he said. “President Trump is poised to trample upon that great legacy with a de facto Muslim ban.”

The Council on American-Islamic Relations will on Monday announce a federal lawsuit on behalf of more than 20 people challenging the constitutionality of the executive order.

“There is no evidence that refugees – the most thoroughly vetted of all people entering our nation – are a threat to national security,” said CAIR national litigation director Lena F. Masri. “This is an order that is based on bigotry, not reality.”

This post has been updated.