Declaring someone who does an act nullifying Islam as a Kafir

Q 2: Can anyone call a Muslim a Kafir (disbeliever) if they witness them doing something that contradicts Islam?

A: Ahl-ul-Sunnah wal-Jama`ah (adherents to the Sunnah and the Muslim mainstream) do not call any Muslim a Kafir for committing a sin, like the Khawarij (separatist group that believes committing a major sin amounts to disbelief) and Mu`tazilah (a deviant Islamic sect claiming that those who commit major sins are in a state between belief and disbelief) do, unless they consider it lawful. The only exception is the people who abandon Salah (Prayer) intentionally; they are considered Kafirs even if they do not deny its obligation according to the most authentic of the scholars' views. There is evidence confirming the Kufr (disbelief) of a person who abandons Salah. Also, they (Ahl-ul-Sunnah wal-Jama`ah) do not believe that Iman (Faith) is enough, and there is no harm in committing sins like Al-Murji'ah (a deviant Islamic sect claiming that Faith is restricted to belief not deeds).The people of the Qiblah (direction faced for Prayer) are those about whom the Prophet (peace be upon him) stated, Anyone who faces our Qiblah during Salah, offers Salah like us and eats our slaughtered animals are Muslims; they have the same rights and obligations as other Muslims have. Related by Al-Bukhari and others in the Hadith of Anas (may Allah be pleased with him)They are Ahl-ul-Sunnah wal-Jama`ah who follow the Manhaj (methodology) of the Messenger of Allah (peace be upon him), his Sahabah (Companions, may Allah be pleased with them) and their followers.However, if a person commits anything that nullifies Islam, they are Kafirs, such as those who seek the help of the dead, slaughter animals for them, abuse Allah and His Messenger, ridicule Islam and such practices clarified by the scholars in the ruling on apostates.Ahl-ul-Sunnah wal-Jama`ah do not testify that someone is a Kafir or doomed, except for a strong reason. It is one of the greatest forms of injustice to testify that someone will not be forgiven by Allah. This was severely warned against. (Part No. 1; Page No. 425) A Muslim must be careful, guard his safety, and refrain from interfering in matters that are not of their concern. It was reported in the Two Sahih (authentic) Books of Hadith (i.e. Al-Bukhari and Muslim) on the authority of Ibn `Umar (may Allah be pleased with him) that the Messenger of Allah (peace be upon him) stated, When a person calls his brother (in Islam) a Kafir, one of them will certainly deserve the title. If the addressee is so as he has asserted, the Kufr of the addressee is confirmed, but if it is untrue, it will revert to him (the sayer). Also, there is another Hadith reported on the authority of Abu Dhar (may Allah be pleased with him), that he heard the Prophet (peace be upon him) stating, If anyone accuses another of Kufr or calls him the enemy of Allah, such an accusation will revert to the accuser if the accused is innocent. It was mentioned in Sahih Muslim in the Hadith reported on the authority of Jundub ibn `Abdullah (may Allah be pleased with him) that the Messenger of Allah (peace be upon him) stated, A man said, "By Allah! Allah will not forgive so-and-so!" Allah (Glorified and Exalted be He) said, "Who is that who takes an oath in My Name that I will not forgive so-and-so? I have forgiven him and rendered your good deeds fruitless." May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.