Short answer
Islam asks for sincere faith, chosen freely, and the Quran says there is no compulsion in religion. Leaving Islam after knowingly accepting it is one of the gravest sins. Having doubts, questions or unwanted thoughts is not leaving Islam: the Prophet ﷺ called hating such thoughts a sign of faith.
Faith is affirmation in the heart with declaration by the tongue. A person leaves Islam only by willingly rejecting something known to be part of it, not by passing doubts, waswasa (intrusive thoughts) or sins. Words said under real coercion do not take a person out of Islam. Legal consequences of apostasy, such as for marriage and inheritance, are matters for a qualified court and mufti, and no individual may take matters into their own hands.
The four schools agree on this.
Explanation
Faith that is forced is not faith. The Quran says the truth has been made clear from error and there is no compulsion in religion, and that each person is free to believe or disbelieve and will answer to Allah for it. That is why Islam puts so much weight on understanding and sincerity.
Doubts are not apostasy. Some Companions told the Prophet ﷺ they found thoughts in themselves too terrible to speak of. He said that their dislike of these thoughts was clear faith. If you are struggling, you are not out of Islam. Keep praying, ask your questions to a knowledgeable and patient teacher, and read about the foundations, such as tawheed and the articles of faith. The four schools agree that doubts one dislikes do not take a person out of Islam.
Leaving deliberately. Rejecting Islam after knowingly accepting it is a grave sin, and the Quran warns of its consequences in the Hereafter for those who open their hearts to disbelief willingly. The door of return stays open as long as a person lives: anyone who left can say the shahada again and be welcomed back.
Classical fiqh discusses apostasy in the law of courts and states, a subject beyond a Q&A page. What matters for individuals and families is this: be patient with someone who is struggling, keep ties with them, never threaten or harm them, and keep the door open. For a personal situation, speak to a trusted scholar.
The evidence
Quran, Surah Al-Baqara 2:256
لَآ إِكْرَاهَ فِى ٱلدِّينِ ۖ قَد تَّبَيَّنَ ٱلرُّشْدُ مِنَ ٱلْغَىِّ ۚ فَمَن يَكْفُرْ بِٱلطَّٰغُوتِ وَيُؤْمِنۢ بِٱللَّهِ فَقَدِ ٱسْتَمْسَكَ بِٱلْعُرْوَةِ ٱلْوُثْقَىٰ لَا ٱنفِصَامَ لَهَا ۗ وَٱللَّهُ سَمِيعٌ عَلِيمٌ ﴿٢٥٦﴾
There shall be no compulsion in [acceptance of] the religion. The right course has become clear from the wrong. So whoever disbelieves in Taghut and believes in Allah has grasped the most trustworthy handhold with no break in it. And Allah is Hearing and Knowing.
Faith must be chosen freely.
Quran, Surah Al-Kahf 18:29
وَقُلِ ٱلْحَقُّ مِن رَّبِّكُمْ ۖ فَمَن شَآءَ فَلْيُؤْمِن وَمَن شَآءَ فَلْيَكْفُرْ ۚ إِنَّآ أَعْتَدْنَا لِلظَّٰلِمِينَ نَارًا أَحَاطَ بِهِمْ سُرَادِقُهَا ۚ وَإِن يَسْتَغِيثُوا۟ يُغَاثُوا۟ بِمَآءٍۢ كَٱلْمُهْلِ يَشْوِى ٱلْوُجُوهَ ۚ بِئْسَ ٱلشَّرَابُ وَسَآءَتْ مُرْتَفَقًا ﴿٢٩﴾
And say, "The truth is from your Lord, so whoever wills - let him believe; and whoever wills - let him disbelieve." Indeed, We have prepared for the wrongdoers a fire whose walls will surround them. And if they call for relief, they will be relieved with water like murky oil, which scalds [their] faces. Wretched is the drink, and evil is the resting place.
Each person is responsible for their own choice before Allah.
Quran, Surah An-Nahl 16:106
مَن كَفَرَ بِٱللَّهِ مِنۢ بَعْدِ إِيمَٰنِهِۦٓ إِلَّا مَنْ أُكْرِهَ وَقَلْبُهُۥ مُطْمَئِنٌّۢ بِٱلْإِيمَٰنِ وَلَٰكِن مَّن شَرَحَ بِٱلْكُفْرِ صَدْرًۭا فَعَلَيْهِمْ غَضَبٌۭ مِّنَ ٱللَّهِ وَلَهُمْ عَذَابٌ عَظِيمٌۭ ﴿١٠٦﴾
Whoever disbelieves in Allah after his belief... except for one who is forced [to renounce his religion] while his heart is secure in faith. But those who [willingly] open their breasts to disbelief, upon them is wrath from Allah, and for them is a great punishment;
The grave warning is for those who choose disbelief willingly, not those under coercion.
Sahih Muslim 132
Sahih... ذاك صريح الإيمان ...
"That is clear faith."
Hating intrusive doubts is a sign of faith, not its loss.
Where the ruling is recorded
- al-Fiqh al-Akbar, the section on faith and its definition. Faith is affirmation and belief, and sins do not remove a person from it.
- Radd al-Muhtar (Ibn Abidin), Kitab al-Jihad, Bab al-Murtadd. A Muslim is not judged to have left Islam when his words can be interpreted in a sound way, and hesitation in judging is required.
This answer is pending review by the scholars on our team. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.