Benefit 70: Implications of apostasy (taken from Sheikh Yahya's "Mukhtaṣar Al-Maqāl"):

1. The apostate is adviced and then requested to repent and return to Islām (الاستتابة), but if they persist upon disbelief they are to be executed as an apostate 2. The ties of faith and loyalty between them and the Muslims are completely severed 3. It is not permissible for them to marry a Muslim 4. It is not permissible for them to act as a waliyy (guardian) for a Muslim relative in marriage 5. It is not permissible for them to inherit from a deceased Muslim relative, nor is it permissible for the relative to inherit from them 6. It is not permissible for them to enter Makkah and the surrounding sanctuary 7. Their slaughtering is impermissible 8. Their testimony is invalid 9. Zakāh is not collected from them nor distributed to them 10. It is not permissible to offer funeral prayer over them 11. It is not permissible to pray behind them 12. It is not permissible to seek forgiveness for them after their death ... After reflecting over these implications, you can see why making Takfeer on Muslims is such a serious matter! You can also see why abandoning prayer (an act of disbelief) is such a grave sin!!!