ANSWER
Alhamdulillaah!
Firstly, it should be known that the default ruling for every person who claims to be Muslim or who adheres by any of the famously known symbols of Islam is that he is muslim until we see in him/her what repudiates Islam and nullifies it.
Secondly, whenever it becomes clear that a muslim has said or done, or believes anything that contradicts and nullifies Islam, it must be understood that before he/she can be declared non-Muslim, some conditions must be fulfilled and some Preventative Factors (Mawaani’) absent. This is agreed upon by the Ulamaa of Sunnah world-over?
Whenever it can be proved that a Muslim has carried out an act which textual evidences has proven to be kufr, and the conditions (shurut) to declare one a Kaafir are complete in him, and the Preventative Factors (mawaani’) are all absent, then such a person is to be declared a Kaafir.
Thirdly, it should be realised that the Shi’ite sect is very diverse and of different denomination.
There are those that the Ulamaa have a consensus to be kaafirs such as the Nusayriyya, the Fatimiyyah, the Ubaydiyya and the Qaraamitah.
Their consensus follows that anybody that attributes himself to such a sect either he is a leader there, a scholar or a mere follower, he is categorically out of the fold of Islam. For these denominations are the ones that believe in the prophethood of Ali or the superiority of Ali over the Rasul – salallaahu alayhi wasallam or even worship Ali – radiyallaahu annhu.
At the same time, there are those shi’ite denominations that the Ulamaa of Sunnah did not declare as totally Kaafirs.
Such as the Yazidiyya
Then there are those about whom the salaf differed on their encompassing ruling. Some declared anybody attributing himself to them as kaafir while others declare their leaders kaafirs and their followers to be Muslims unless when they stick to the creed of their leaders. The main denomination of the Shi’a today: the “Ithnay Ash’ariyya” are of this category.
This is because there are over 22 denominations of Shi’ite creed as Imaam Al-Isfarayini – rahimahullaah – put it in his ‘Sharhu Al-Aqidah’
Though the questioner did not specify, we think he is referring to the ‘ithnay ashariyya’ (twelvers).
As for them, the right verdict – which is the verdict of the Muhaqqiqun among the Ulamaa – is that the leaders and du’at among them who hold views such as the kufr of the sahaabah, the incompleteness of the Qur’an, ad who insult and curse the sahaabah such as Abubakr and Umar and A’ishah – radiyallaahu anhum – are -Kaafirs.
But, as for their commoners who were either born into it or just attributing to it by identity and lack the understanding of it and do not hold its creed, then they should not be flatly regarded as Kaafirs until such a confirmation is made about them.
What we have explained here is the verdict of the Ulamaa such as Ibn Hazm al-Andalusi – rahimahullah – in his book “Al-Fasl Fi Al-Milali wa Al-Ahwaa’i wa An-Nihali”; Ibn Taymiyya in his “Minhaaju As-Sunnah”, “Majmu’atu Al-Fataawah” and “Iqtidaa Siraat Al-Mustaqim”, Abu Ishaq Ash-Shaatibi – rahimahullah – in his “Al-I’tisaam”; and among the Ulamaa of today, Al-Maqdisi in his “Waqafaat”, and Shaykh Ad-Dadw in his “Shar’h Kitaabu At-Tawhid”, and many others.
We do not dispute the fact that the Shi’ite creed is of the most dangerous against Islam and the Muslims.
Regarding that, Ibn Taymiyyah – rahimahullaah – said:
“As for the Rawaafid (Shi’ites), then they are worse in bid’a than the khawaarij, and they declare as kaafirs those whom even the khawaarij do not declare as kaafirs for the khawaarij did not declare as kaafir Abu bakr and Umar. And they (shiites) lie on the Prophet -salallaahu alayhi wasallam – and the companions a sort of lie that no one lied. The Khawaarij did not used to lie but the khawaarij were more truthful and brave than they were, and more fulfilling of the covenant they took. Thus they (the khawaarij) were more oft in waging (Jihad) than they (the shi’ites) were. These ones (the shi’ites) are worse liars, worse cowards, worse cheats and lowly in character (than anyone besides them), and they even seek the assistance of the kuffaar over Muslims…”
And it is known in our history that the Muslim Ummah had always been defeated by the kuffar during invasions only with the cooperation and alliance of the Shi’ites.
If we have been able, thereby to answer your question, alhamdulillah, but if you need more clarification, then you can still raise questions.
And Allah knows best