In this connection, I would like to narrate one incident that happened during the conquest of Mecca. When the Prophet Muhammad (known as "mercy for the universe" in the Qur'ān) marched into the city of Mecca in the 8th year of hijra, he declared a general amnesty for all his enemies. However, the same Prophet named seven or eleven persons "who should be killed even if they are found holding on to the cover of the Ka`bah!"21 Those who like to understand or evaluate Islam from the secular/humanist point of view must realize that Islam should be judged on its own terms, and not by the secular ideals.
Out of those seven persons, one case is interesting and relevant to our discussion. It is an example of a murtad milli--a non-Muslim who became Muslim and then became kāfir again. His name was `Abdullāh bin Sa`d bin Abi Sarah, a foster-brother of `Uthmān bin `Affān. He had come to Medina and professed Islam, then he went back to Mecca and become a kāfir again.
In spite of the Prophet's order to kill `Abdullāh, `Uthmān sheltered him till after the conquest, and then brought him to the Prophet and asked for forgiveness. The Prophet remained quiet for a while hoping (as he himself said later on) that someone would stand up and implement his standing order by killing `Abdullāh. But when no one understood the meaning of his silence, the Prophet granted pardon to him.22
This is an example of murtad milli (a naturalized Muslim who reverts to kufr) who is to be given the chance for repenting; and if he repents, then he is not to be killed. This is exactly what the Sh`iah jurists also say.
One should also realize that by the time of the Prophet's death, most Muslims were "naturalized Muslims;" and a vast majority of those who were "Muslim by birth" had not yet reached adulthood. So looking for an example of a murtad fitri during the Prophet's lifetime would be unrealistic.
Notes:
21. See any comprehensive work on the life of the Prophet likeTa'rākh Abi 'l-Fidā and al-Khamās.
22. Ibid