U.S. Secretary of State John Kerry accused ISIS militants of apostasy this week. Above, Kerry speaks about the Paris attacks and ISIS at the Overseas Security Advisory Council's 30th annual briefing at the State Department in Washington, Nov. 18, 2015. Reuters/Kevin Lamarque

U.S. Secretary of State John Kerry waded into a heated Islamic theological debate this week when he chose to label the Islamic State group “apostates,” a term used to denote those who have left the fold of Islam, Agence France-Presse reported Tuesday. The charge of apostasy is punishable by death in several majority-Muslim countries, and Orthodox theologians have typically shied away from using the accusation against those who claim to be believers.

“Daesh is in fact nothing more than a mixture of killers, of kidnappers, of criminals, of thugs, of adventurers, of smugglers and thieves,” Kerry said, using an Arabic acronym to which the terrorist organization, aka ISIS, takes offense. “And they are also, above all, apostates — people who have hijacked a great religion and lie about its real meaning and lie about its purpose and deceive people in order to fight for their purposes.”

ISIS claims to have established an Islamic state based on the Quran and prophetic example, and often labels its enemies apostates, including those who practice Islam differently than they do, like Shiites. Muslim scholars have widely condemned the group since it arose in Syria and Iraq in 2014 but have generally stopped short of using that terminology for the extremist group.

President Barack Obama has said the organization is not Islamic, an assertion meant to distance more than a billion Muslims from the radical group and that has brought him criticism from Republicans. He has never raised the accusation of apostasy, however.

“ISIL is not Islamic. No religion condones the killing of innocents, and the vast majority of ISIL's victims have been Muslim,” Obama said in September 2014, using yet another name for ISIS. "ISIL is a terrorist organization, pure and simple, and it has no vision other than the slaughter of all who stand in its way."

The terrorist group continues to rule over large swaths of Iraq and Syria but has seen its grip loosen on a number of provinces in recent months. The U.S. has been engaged in an aggressive airstrike campaign against ISIS since the summer of 2014.