Salafi (Islamist) extremism, embraced by organizations such as the Islamic State, is a global phenomenon whose primary cause is not clearly elucidated in the literature. Unlike most existing models or explanations of terrorism that attempt to explain stages or factors associated with the radicalization of individuals, the proposed theory aims to identify the dominant causal pathway behind the onset of Salafi extremism. Kosovo was used as a control group to establish Saudi Arabia-originated Wahhabism's dominance as a radicalization source. The theory's independent, dependent, and causal variables are public support levels for religious leaders as religious judges, the radical agenda they espouse, and the idea of Sharia as the law of the land, respectively. For nearly 20 nations listed in the 2013 Pew survey report titled "The World's Muslims: Religion, Politics and Society," the computed coefficients of determination for the public support levels for religious leaders and a radical agenda, Sharia and religious leaders, and Sharia and a radical agenda, were 0.83, 0.85, and 0.86, respectively, using linear regression methodology. The belief in Sharia as the law of the land, as advocated by religious leaders, provides the basis for radicalization and the emergence of Salafi jihadist groups.