Now dozens of Islamic people have taken to social media to warn that declaring someone a non-Muslim is forbidden by ancient Islamic legal guidelines.
The act of describing someone as an apostate (someone who has left the religion) is called " takfir " in Arabic.
Some scholars believe this is so grave a sin that it is punishable by death, although different schools of Islam differ on this point.
The concept of Takfir dates back to a hadith (sayings of the Prophet Mohammed) in which he said: "He who says to his brother 'O disbeliever', then it returns upon one of them."
It is important to understand that a Muslim who criticises anyone using the hashtag does not necessarily support the crime, but is concerned about the dangers of excommunicating anyone from the faith.
The so-called Islamic State have drawn criticism across the Muslim world because they repeatedly justify their atrocities against members of the same religion by describing them as apostates.
Bilal Ahmed, a writer and activist, wrote a blog post explaining the religious concerns around the practice of takfir .
"For all the relief that we may feel at someone taking care not to automatically associate Islam with terrorism, I am deeply alarmed at the fact that we feel this comfortable engaging in takfir ," he wrote.
"Takfir is a term that essentially describes situations where a Muslim’s legitimacy is being questioned.
"It refers to a practice of exclusion, whereby a Muslim is declared to be somehow impure, and an illegitimate member of the religious community.
"These days, it is used to refer to salafi [fundamentalist] and militaristic strains of Islamic practice, which rely on moralist fervour about legitimate and illegitimate Muslims."