Ibn Juzayy al-Kalbī, the Mālikī jurist, compiled a list of twenty actions that will cause a person to be out of the fold of Islam.¹ This is a scholarly opinion, so it is not exhaustive, but certainly universal.

1. Negating that Allah has Lordship over His creation
2. Negating that Allah is one in His entity, attributes, and actions
3. Worshiping another entity along with God
4. Changing one’s religion to other than Islām after having learned about Islām and understanding it
5. Claiming that God can become manifest in His creation (e.g. take the form of a man)
6. Believing in reincarnation
7. Negating any one of God’s known attributes. Included in this is claiming that the Universe was created by other than Him or that He was born from something else. Included in this also is claiming that the Universe had no beginning in time
8. Claiming that one has sat alongside with Allah literally speaking or claiming that one has ascended to visit Him
9. Claiming that a person after the time of Prophet Muhammad ﷺ is a real prophet from Allah (and that Prophet Muhammad ﷺ is not the last prophet). Included in this is claiming that one has received revelation from God (like a prophet)
10. Stating that it is possible that the prophets lied to us
11. Claiming that the message of Islām is only for Arabs (or only for another chosen group)
12. Claiming that one will enter Paradise (literally) while still in this world
13. Claiming that the punishment and reward in the next life is only confined to being metaphorical.
14. Calling all of the Companions of the Prophet ﷺ (all together) disbelievers
15. Denying any of the necessarily known and obvious parts of the religion (e.g. claiming that formal prayer, fasting in Ramadan, Zakat, and Hajj is not obligatory)
16. Claiming that there is no need to worship Allah externally any more after becoming spiritually advanced. For example, claiming that the formal prayer is no longer obligatory after one has reached some high spiritual station with God
17. Denying any part of the Qur’an left by Prophet Muhammad ﷺ
18. Intentionally adding to (inserting one’s own words into) or changing any part of the Qur’an left by the Prophet ﷺ [Included in this is fabricating obligatory tenets of belief or obligatory acts of worship which have no basis in the primary texts]
19. Claiming that others besides God could produce the Qur’an
20. Claiming that the latter scholars/saints (e.g. Imām Mālik, Imām ash-Shāfi`ī, Shaykh ‘Abdul Qādir Jilānī, etc.) were better than the prophets

Also see: What Takes a Person Out of the Fold of Islam?

1. Al-Qawanin al-Fiqhiyyah, page 323, lines 7-18