Assalamualikum warahmatullahi wabarakatuhu. If a shia believes that the Quran has been altered, curses the companions of the Prophet because he has wasn't clarified the truth and he has weak Islamic Knowledge. In such a case is the this shia declared Kafir or it is decided by the judge? Jazakallahukhair.

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

Believing that the Quran has been distorted is disbelief that contradicts what Allah has informed us about His Preservation of His Book. Similarly, insulting the companions in some cases is also disbelief.

However, a specific person cannot be declared a Kaafir (disbeliever) unless after the evidence is established against him as Shaykh Ibn Taymiyyah has established in some of his statements.

The issue of Takfeer (declaring a person as a non-Muslim) and judging a person as a non-Muslim is a matter that can only be decided by the judges and scholars, not the common people, because there are some conditions that must be met for declaring a person as a non-Muslim while there should be no impediments that prevent from declaring him as such, and it is only the Muslim judge or the trustworthy scholar who can verify all of this.

For more benefit on declaring someone as a non-Muslim, please refer to Fataawa 91677, 87963, 91777, 91372 and 133655.

For more benefit on insulting the Companions, please refer to Fatwa 2429.

Allah Knows best.