This week a Canadian Muslim gunman went on a rampage in Ottawa, killing a soldier and storming into the Parliament building before he was shot dead. Authorities have since said he had applied for a passport to travel to Syria. Three Muslim schoolgirls from Colorado were intercepted in Germany apparently on their way to Syria, the base for attacks there and in Iraq by the terror group Islamic State, or ISIS. An Aug. 20 article in Newsweek estimated that perhaps twice as many British Muslims are fighting for ISIS as are serving in the British army.

What could possibly inspire young Muslims in the West to abandon their suburban middle-class existence and join a holy war? How could teenagers in Denver or anywhere be lured by a jihadist ideology—or are grisly videos of ISIS beheadings and crucifixions not enough of a deterrent?

What is so compelling about radical Islamism may lie within its founding texts. It is time we acknowledged the powerful influence these texts have had even on ordinary Muslims. The political ideology based on them has already dragged the Middle East back toward the Stone Age.

As a teenager growing up in Egypt in the 1980s, I liked to stroll through Cairo’s outdoor book market, fishing out little gems like an Arabic translation of “War and Peace.” One day I stumbled upon a book that shook everything I believed in.

The book was “In the Shadows of the Quran,” Sayyed Qutb’s magnum opus. The Egyptian writer, who died in 1966, remains arguably the most influential thinker in contemporary Muslim societies. He was the principal theorist of the Muslim Brotherhood and the intellectual impetus behind the Islamist parties it spawned. Qutb’s ardent disciples included Osama bin Laden and Ayman Zawahiri of al Qaeda. It is not an exaggeration to say that Qutb is to Islamism what Karl Marx is to communism.