Conditions of personal Takfir
The concept of excommunication in Islam has conditions. This form of Takfir is to be done on the observable. If someone were to commit major Shirk, then he is not to be excused for being seen as a disbeliever. The muslim judges on what he observes.
Because of our natural inclination (Fitrah), it is impossible to be ignorant of it. Allah bestowed on us basic knowledge of Tawheed and Shirk.
'Umar bin Al-Khattab reported:
"People were (sometimes) judged by the revealing of a Divine Inspiration during the lifetime of Allah’s Apostle but now there is no longer any more (new revelation). Now we judge you by the deeds you practice publicly, so we will trust and favor the one who does good deeds in front of us, and we will not call him to account about what he is really doing in secret, for Allah will judge him for that; but we will not trust or believe the one who presents to us with an evil deed even if he claims that his intentions were good."
(Sahih Bukhari Book 48 Hadith 809)
Not all deeds are known by the Fitrah. Issues of haram and halal are known through revelation. But Tawheed always stays the same. There are deeds inside the legislation that amount to major Kufr.
The Fitrah is also ignorant of them. Take for example the major Kufr deed of leaving the prayer. Not all major Kufr deeds get the excuse of ignorance. For example, someone cannot be ignorant to know insulting Allah or the prophet is disbelief.
Imam Shafi'i said:
“Allah has Names and Attributes which are mentioned in His Book, and which His Prophet (peace be upon him) narrated his Ummah. No one who has been given clear evidence can reject that, because they are mentioned in the Quran and are mentioned in the Sahih narrations of the Messenger of Allah (peace be upon him).
So if a person goes against that after being given clear evidence, then he is a disbeliever. However, before clear evidence is presented to him, he is excused for his ignorance of it, because knowledge of such matters is not known on the basis of reasoning or on the basis of reflection. We do not consider anyone to be a disbeliever because he does not know this, except after knowledge of it reaches him.”
[Tabaqat al-Hanabilah 1/283]