Why is it not permissible for a Muslim to convert to another religion, especially if he was born a Muslim, and is the punishment for apostasy not mentioned in the Qur’an?

The answer can be summarized as follows: Apostasy is a turning back from Islam. Whoever dies upon it, their deeds are nullified, and they will abide eternally in Hellfire. The apostate is asked to repent; if they repent, well and good, otherwise, they are executed due to their disbelief, by scholarly consensus. The ruling on apostasy does not contradict the prohibition against compelling people to convert to Islam, as this prohibition does not include the apostate.

Children of Muslims are considered Muslim, following their parents. It is valid for a discerning child to embrace Islam, but their apostasy is a matter of scholarly disagreement. If a discerning child apostatizes, they are not executed according to the majority of scholars; instead, their attainment of puberty is awaited.

If a child born to Muslim parents reaches puberty and does not affirm Islam, their fate is a matter of disagreement: Hanbalis consider them an apostate and they are asked to repent; if they repent, well and good, otherwise, they are executed. However, Hanafis and Shafi'is do not consider them an apostate; rather, they are compelled to embrace Islam through imprisonment and corporal punishment, or efforts are made to guide them.

The disagreement in this matter pertains to judicial ruling, but the disbelief itself is agreed upon. Mercy towards the apostate lies in saving them from eternal dwelling in the fire of Hell.