A Mosque as Extremist Megaphone

President Obama on Wednesday will visit a U.S. mosque for the first time in his presidency. According to the White House, during this visit he will “celebrate the contributions Muslim Americans make to our nation and reaffirm the importance of religious freedom to our way of life.” Over the past two years, in the president’s efforts to counter violent extremism, he has emphasized the responsibility of Muslim “scholars and clerics” to help ensure that mosques are not used as a platform to preach Islamist extremism.

Such extremism isn’t limited to out-of-the-way mosques where radical clerics operate in the shadows. It is occurring in mainstream and leading mosques world-wide, including at one of the most important religious institutions in Islam, the Al Aqsa mosque in Jerusalem.

Consider a Jan. 16 sermon at Al Aqsa by Sheikh Abu Taqi Al-Din Al-Dari, a Palestinian cleric who called for jihad against the West and Europe, and for the burgeoning Islamic State to “conquer Rome, Washington and Paris.”

Despite Al Aqsa’s importance to Islam—it is considered the religion’s holiest site outside Saudi Arabia—few Westerners are aware of the content of the sermons, lectures and lessons offered there. Many of these sermons are posted on the mosque’s two official YouTube channels and have been translated from the Arabic by my organization, the Middle East Media Research Institute.

What we have found at Al Aqsa is a steady stream of calls for jihad and martyrdom, venomous attacks on Jews, Christians and other non-Muslims, and praise for al Qaeda, Islamic State, or ISIS, and other jihadist groups.

Calls for the destruction of the U.S. and the West, including promises that Islam will take over the world, are other common themes. On July 24 last year, Sheikh Ahmad Al-Dweik—a frequent lecturer at the mosque and Palestinian cleric, like the other religious leaders quoted here—said: “The caliphate will come to be, and the nuclear bomb will be produced,” adding that this future Islamic caliphate—will “fight the U.S. and will bring it down” and “eliminate the West in its entirety.”

On July 6, 2015, Sheikh Muhammad Abed, known as “ Abu Abdallah,” declared that from “the land of the Prophet’s nocturnal journey”—a reference to Jerusalem—“armies will set out to conquer Rome, to conquer Constantinople,” and then he added to the list “Washington and London.”

In an Oct. 27 address at Al Aqsa, Sheikh Khaled Al-Maghrabi called for the annihilation of the Jews all over the world, providing justification by quoting the well-known hadith (sayings of the Prophet Muhammad) of the stone and the tree: “Oh Muslim there is a Jew behind me, come and kill him.” Earlier at the mosque, on May 29, Sheikh Al-Maghrabi explained why Jews were killed in the Holocaust. “On Passover,” he said, the Jews “would knead the dough for these matzos with children’s blood. When this was discovered, the Israelites were expelled across Europe . . . It got to the point where they were burned in Germany.”

Such anti-Semitism is familiar at Al Aqsa. In another sermon there, on Nov. 28, 2014, cleric Omar Abu Sara called the Jews—to whom he said “every single vile trait has been attributed”—the “most evil creatures to have walked this Earth.” He added that “it was the Jews whom Allah turned into apes and pigs.”

If incendiary sermons such as those at Al Aqsa were being delivered in any Western city, authorities would not tolerate them. Over the past year in Europe, several sheikhs have been prosecuted or expelled for similarly extremist rhetoric. In Germany, an imam from Denmark who spoke in 2014 at the Al Nur mosque in Berlin faced criminal charges after calling for the killing of Jews, saying: “Count them and kill them to the very last one.” Two months ago, he was fined €1,300 ($1,420). Italian, Spanish and Danish authorities have handled similar matters involving hateful sermons.

At Al Aqsa, support is also strong for jihadist groups. On June 26 last summer, Palestinian cleric Issam Amira praised an ISIS-inspired attack in the small French town of Saint-Quentin-Fallavier, southeast of Lyon, after an Islamist radical decapitated his employer and then crashed his vehicle, causing an explosion. “The Muslims have given rise to ISIS, and to vehicular attackers,” the cleric said. “They are courageous.” He added his wish that “Allah soon reward the Islamic nation with a second rightly guided caliphate.” Destroying “the Jewish entity”—Israel—could then be achieved, he said.

Al Qaeda also comes in for praise at Al Aqsa. The day Osama bin Laden was killed, May 2, 2011, an unnamed preacher eulogized him. In an Al Aqsa video uploaded to YouTube, the speaker vowed that despite bin Laden’s death, the “nation of one billion Muslims will give birth to hundreds of millions” of bin Ladens. Then came a threat to President Obama: “You personally gave the order to kill Muslims. . . . The day will soon come when you find yourself hanging from the gallows.”

Our research at Memri indicates that the hateful rhetoric on display at Al Aqsa is hardly unique to that institution—similar incitements can be found at prominent mosques and Islamic institutes throughout the world. Our video archive at memritv.org contains thousands of examples.

A year ago in Washington, D.C., at the opening of the Summit on Countering Violent Extremism, President Obama urged Muslims to “push back” against extremism in mosques. He will have an opportunity to restate that message on Wednesday in Baltimore. Acknowledging the Islamist threat, and those who support it, must be part of any realistic attempt at countering violent extremism.

Mr. Stalinsky is the executive director of the Middle East Media Research Institute (Memri).