Question Summary:
What is the ruling for a Muslim who becomes an atheist?

Question Detail:

I know someone at university who was brought up a sunni Muslim but after starting university he has become atheist. I would like to know what the ruling on this is e.g how much of a major sin this is and what punishment he will get etc.  Any ruling on this matter would be grately appreciated.

Answer :

In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh.
 
The matter of becoming an apostate is an extremely severe one. Consider the following verse:
وَمَن يَرۡتَدِدۡ مِنكُمۡ عَن دِينِهِۦ فَيَمُتۡ وَهُوَ ڪَافِرٌ۬ فَأُوْلَـٰٓٮِٕكَ حَبِطَتۡ أَعۡمَـٰلُهُمۡ فِى ٱلدُّنۡيَا وَٱلۡأَخِرَةِ‌ۖ وَأُوْلَـٰٓٮِٕكَ أَصۡحَـٰبُ ٱلنَّارِ‌ۖ هُمۡ فِيهَا خَـٰلِدُونَ ( ٢١٧ )
And whoever of you reverts from his religion [to disbelief] and dies while he is a disbeliever - for those, their deeds have become worthless in this world and the Hereafter, and those are the companions of the Fire, they will abide therein eternally. (217) – Surah al-Baqarah
As his friend, it is your duty to bring him back to the fold of Islam. Find out what prompted him to leave Islam in the first place. Consult with the local Ulamaa and seek their advice. If your friend has unanswered questions about Islam or any misunderstandings, you must clear them with the aid and guidance of the Ulamaa. By way of example, if you woke up one morning to find out that there was a fire underneath the bed of your friend, wouldn’t you do anything to get him out of bed? Just imagine how severe of a matter it is if the fire of Hell threatened your friend! You would help him escape at any cost. May Allah bless you for your effort and concern and save your friend from the fire of Hell. Aameen.
 
And Allah Ta’āla Knows Best
Muhammad Haris Siddiqui
Student Darul Iftaa
Melbourne, Australia 
Checked and Approved by,
Mufti Ebrahim Desai.