The Arabic word murtadd applies to a Muslim who publicly declares kufr or unbelief — we seek refuge in Allah — by so stating or by denying a fundamental tenet of creed — such as Allah’s angels — or definitely abandoning or denying a fundamental obligation in the Religion such as prayer (salaat), or acting in a way unmistakably tantamount to unbelief such as prostrating to an idol.
The Law considers irtidad not an exercise of personal freedom — as the West construes atheism and “agnosticism” in its own societies — but a crime and declaration of war against Muslim society, which is duty-bound to protect itself from its consequences. This is a condition of irtidad as established by the Prophet’s statement — Allah bless and greet him:
“The blood of a Muslim is illicit to shed except for one of three reasons: a married adulterer must be stoned; one who wilfully commits murder must be put to death; and a man who comes out of Islam and fights Allah and His Prophet must be put to death or crucified or banished from the earth.”
It is narrated with transmission chains of trustworthy narrators from `A’isha by al-Nasa’i and Abu Dawud in their Sunan and al-Bayhaqi in al-Sunan al-Kubra. This hadith explicitly makes “fighting Allah and His Prophet,” i.e. fighting Islam and Muslims, a condition sine qua non for punishability by death. For then the murtadd becomes the same as an enemy soldier in wartime (muharab), i.e. an enemy of Islam on the battlefield.