From Foreign Affairs:
When ISIS militants hold up a single index finger on their right hands, they are alluding to the tawhid, the belief in the oneness of God and a key component of the Muslim religion. The tawhid comprises the first half of the shahada, which is an affirmation of faith, one of the five pillars of Islam, and a component of daily prayers: “There is no god but Allah, Muhammad is the messenger of Allah.”
It is no surprise, then, that the shahada features prominently in ISIS’ public image. The group’s black flag bears the vow’s words in white Arabic script (as does Hamas’ and even Saudi Arabia’s). And Muslims have long associated a single index finger with the shahada in a variety of contexts, ranging from daily prayers to conversions.
But for ISIS, the symbol is more sinister than a mere declaration of monotheistic beliefs. As Salafi jihadists, members of the group adhere to a fundamentalist interpretation of tawhid that rejects non-fundamentalist regimes as idolatrous. In other words, the concept of tawhid is central to ISIS’ violent and uncompromising posture toward its opponents, both in the Middle East and in the West.
When ISIS militants display the sign, to one another or to a photographer, they are actively reaffirming their dedication to that ideology, whose underlying principle demands the destruction of the West. If rank-and-file soldiers are aware of the precise theological implications of their sign -- and it would be no surprise if they are -- that would be a sobering comment on their deep-seated opposition to pluralism.
The gesture is equally important for what it means to Westerners, most of whom cannot read Arabic. By raising their index fingers, militants send an easy-to-understand message of the group’s goals of theological supremacy and military hegemony. When potential ISIS recruits in London, New York, or Sydney see the symbol on Twitter, they can grasp the scale of ISIS’ ambitions and its underlying aims. At some visceral level, less-radicalized viewers understand that it means dominance.