The barbaric murder of an elderly French priest Tuesday shows ISIS has made good on its chilling two-year-old pledge to bring its war on the West into the very sanctuaries of Christianity, experts told FoxNews.com.

Shouting "Allahu Akbar," the radical Islamist killers slit the throat of 85-year-old Jacques Hamel and critically wounded one other person during the morning attack inside a Catholic church near the Normandy city of Rouen. The terrorists, who forced Hamel to kneel before they slaughtered him, were later killed by police marksmen.

"They forced him to his knees. He wanted to defend himself. And that's when the tragedy happened," said the nun, identified as Sister Danielle.

ISIS' Amaq news agency said the France attack was carried out by two Islamic State "soldiers," Reuters reported.

"[ISIS] has declared war on us," French President Francois Hollande said Tuesday. "We must fight this war by all means, while respecting the rule of law -- what makes us a democracy."

For two years, the black-clad jihadist army has called for attacks on Christians in Rome, throughout Europe and across the world. It has even called for the assassination of Pope Francis. The attack -- which the knife-wielding ISIS killers reportedly videotaped -- in the northern French town of Saint-Etienne-du-Rouvray shows Islamist killers have heeded the call.

“The Islamic State is persistently demoralizing European unity by launching divisive attacks within its borders -- the most recent attack on the Catholic Church aims directly at the French sense of identity,” said Veryan Khan, editorial director for the U.S.-based Terrorism Research & Analysis Consortium.

Calls for attacks on Europe in general and the Catholic Church in particular have built to a fever pitch over the last several months, Khan said. Two months ago, ISIS circulated a video message on the social media site Telegram about migrations to the Caliphate from within the Middle East and from Europe. In the video, “the Pope is demonized and the land of the Crusaders is lambasted,” Khan said.

"Truly, we will fight you even in your churches until we raise there that the name of Allah is the only God," read one propaganda image recently released by ISIS, and depicting a fighter waving a flag atop a European church.

Over the weekend, ISIS Twitter accounts called for more operatives to take up arms in France and carry out additional deadly attacks, according to an analyst with the U.S. based company GiPEC.

Vatican spokesman the Rev. Federico Lombardi on Tuesday said the attack is especially evil "because this horrific violence took place in a church, a sacred place in which the love of God is announced, and the barbaric murder of a priest and the involvement of the faithful."

Lombardi called the attack "more terrible news, that adds to a series of violence in these days that have left us upset, creating immense pain and worry."

Pope Francis has expressed "pain and horror for this absurd violence, with the strongest condemnation for every form of hatred and prayer for those affected."

The church was reportedly on a "hit list" discovered at the residence of a would-be ISIS attacker in April 2015, The Sun reported. Sid Ghlam was believed to be planning "imminent attacks" in France when investigators arrested him. Officials allegedly uncovered an arsenal of weapons and found that Ghlam was talking with someone in Syria who had ordered him to strike specific churches -- including the one in Saint-Etienne-du-Rouvray.

One of Tuesday's attackers was on the radar of French police and had traveled to Turkey, said Mohammed Karabila, president of the Regional Council of the Muslim Faith for Haute-Normandie.

ISIS’ onslaught against Christianity has long been in full effect in the Middle East, where the Caliphate’s soldiers force Christians to convert or die and have destroyed churches, tombs and sacred artifacts that date back to Christ’s time. Militants went on a rampage through Iraq's Mosul Museum in February 2015 using pickaxes, sledgehammers, and explosives to forever destroy historic and irreplaceable Christian shrines. In August 2015, the Islamic State devastated an ancient Christian monastery, the Mar Elian monastery, in central Syria, according to the Syrian Observatory for Human Rights.

Last year, ISIS proudly showcased the mass beheading of Egyptian Coptic Christian fishermen in Libya, and in the video that captured the horrific acts, also called for attacks on Pope Francis.

Tuesday’s monstrous attack came just 12 days after Mohamed Bouhlel, claiming allegiance to ISIS, plowed a truck into a crowd attending a Bastille Day celebration in Nice, killing 84.