“But whoever kills a believer intentionally – his recompense is Hell, wherein he will abide eternally, and Allah has become angry with him and has cursed him and has prepared for him a great punishment.” Qur’an (Sahih international English translation)
It is widely accepted in Islam that there is not even one sin, which when committed, will take you out of the fold of Islam and cause you to be branded a kafir or disbeliever. This includes sins such as drinking alcohol, fornication, adultery, as well as other major sins. Rather it is ones disbelief, which as the name implies, causes one to become a disbeliever. An example would be saying that alcohol is allowed, and denying the verses Allah has revealed in the Quran regarding the prohibition of intoxicants. In this case the person has made halal what Allah has made haram, and rightfully deserves the title of kafir.