Is a person sinful for not showing mercy to or remembering his grandfather, due to his belief that he insulted the Messenger of Allah, peace and blessings be upon him, in his death throes? And is this judgment of apostasy correct? And if he was an apostate, is it permissible to name a child after him or to inherit from him?

The default is that a Muslim remains upon their Islam until there is certain proof of what removes them from it. If words of disbelief are uttered by a dying person, the default assumption is that they were due to loss of intellect and unconsciousness, as a person who has lost their intellect is not held accountable. Such a person is treated like other deceased Muslims; it is permissible to pray for mercy upon them, and inheritance between them and their heirs is established. However, if their full consciousness and awareness are confirmed, and the vile words are repeated by them, then a ruling of disbelief can be made, and mercy should not be prayed for them.

Secondly: There is no objection to being named after non-Muslim fathers. The establishment of apostasy and disbelief does not prevent the establishment of lineage. The Companions were attributed to their non-Muslim fathers, and the Prophet, peace and blessings be upon him, said: "I am the son of Abdul-Muttalib," even though his grandfather was upon the religion of his people.

Thirdly: The majority of jurists hold that a Muslim does not inherit from an apostate, based on the Prophet's saying, peace and blessings be upon him: "A Muslim does not inherit from a disbeliever, nor does a disbeliever inherit from a Muslim." If their apostasy is confirmed, their wealth goes to the public treasury (Bayt al-Mal). Some scholars, however, are of the opinion that the wealth goes to their Muslim heir. This view is strengthened if the public treasury is not properly organized, or if the apostate was in non-Muslim lands.