The Sahih Collection of al-BukhariChapter 10: The Qiblaby Imam Bukhari
Translated by: Ustadha Aisha Bewley
384. It is related that Anas ibn Malik said that the Messenger of Allah, may Allah bless him and grant him peace, said, "Whoever prays our prayer and faces our qibla and eats what we slaughter, he is a Muslim and is under the protection of Allah and the protection of His Messenger. Do not act treacherously against Allah with respect to those under His protection."
Who do we call a Muslim? What does Kufar really mean?
Kufar has almost turned into a curse word in our current community setting. People use this in anger towards one another and call them Kufar. When someone is seen smoking or being too close to the opposite gender, Kufar is an easy word off of many people's tongues.
Kufar literally means "unbeliever" and unbeliever is defined as "someone who refuses to believe (as in a divinity." Notice how the word "belief" is the core component, not "action." Of course as Muslims, we believe that belief is shown through action, but as I'm sure anyone can relate to, our actions sometimes are not as Muslim as our heart may be. By definition, there are only two groups - believer and unbeliever. Or, Muslim and Kufar, respectively.
Since only Allah is able to see what is in our heart, we must judge by actions who is a Muslim and who is a Kufar. We judge by outward appearances, but Allah judges by inward realities. We can not go to extremes and call someone a Kufar based on our witnessing of one sin. On the other side, we can not say that all sins are okay as long as you profess the shahadah (There is no god except Allah). Saying one is a Kufar does not, and should not be, labeling a person to Hellfire just like labeling someone a Muslim does not ensure their entry into Paradise.
When we say someone is Kufar, it is simply a legal statement and a label for who should be treated as a Muslim when they get married, die, have kids, etc.
Two terms are important in this discussion:
Tasdiq: "You are telling the truth"
Takdhib: "You are lying"
(both in reference to the Sunnah and Qur'an)
When someone is denying a pillar of Islam, such as saying that you don't have to pray, they are essentially saying to the Prophet Muhammed (peace be upon him), "You are lying" and they are committing tasdiq. To deny a pillar of Islam is the gravest of errors and anything that a person rejects that is necessarily known of the religion will remove them from Islam. For instance, if a person states that drinking is not forbidden, they have committed tasdiq and it is safe to say they are Kufar. Again, does this mean we are saying they are going to Hellfire? No! We do not know what will happen in their life or what is in their heart but this person isn't required to have a nikkah when they get married, they are not expected to pay zakat, etc.
Notice in the examples given that we mentioned people who are denying that an action is a sin or that something is obligatory. There is a big difference between someone committing a sin when they are weak and denying it is a sin altogether. We are all sinners and Muslims may be punished for their deeds, in this life or the next, even if they are eventually destined for Paradise, by Allah's Mercy.
In short, Kufar is a legal term used for anyone who does not appear to be Muslim. Kufar is not a word used for a sinning Muslim, nor is it a derogatory term. We use it to classify someone, especially in legal matters such as paying zakat (required charity tax), creating a marriage contract, etc. It does not mean this person is going to Hellfire as no one in creation can determine this. It is up to Allah.