Beliefs dwell in the heart, but undeniably appear in one’s practices. Thus, Iman (the Islamic faith) manifests itself in the Muslim’s practices which usually reveal the degree of his faith. There are many inward and outward factors that contribute to its increase or decrease. Certain beliefs and acts contradict Islamic faith and thus a Muslim has to be aware of and totally estrange himself from being involved in them.

Responding to this question, AMJA online Jurisprudence Section, stated:

The nullifiers of faith (nawaqidal-iman) are those things that invalidate, falsify and negate it. Apostasy nullifies faith just as ablution (wudu) is nullified by ritual impurity.

The most important nullifiers of faith are:

1 – Associating partners with Allah in Lordship

2 – Associating partners with Allah in worship

3 – Associating partners with Allah in His Divine Names and Attributes

4 – Objecting to, looking down to or denial, rejection or making mockery of the rules of Sharia.

A person who nullifies his faith exits the fold of Islam, if he insists on his attitude after being informed and after the proof has been made clear to him.

Consequently, his marriage contract becomes annulled, and therefore, his wife becomes no longer lawful for him after his apostasy. If he dies on disbelief, all his deeds will be in vain as Allah the Most High said, (And whosoever of you turns back from his religion and dies as a disbeliever, the deeds of these people will be lost in this life and in the Hereafter, and they will be the dwellers of the Fire; they will abide therein forever.) (Al-Baqarah 2:217)
Moreover, his repentance will not be accepted once death has approached him, as Allah the Most High said, (Verily, for those who disbelieved after their belief, and then went on increasing in their disbelief – never will their repentance be accepted [because they repent only with their tongues, not with their hearts]. And they are those who are astray.) (Aal-Imran 3:90)

Furthermore, there is no inheritance between such a person and his Muslim family and relatives as the Prophet (peace and blessings be upon him) said; “A Muslim does not inherit from a disbeliever, nor does a disbeliever inherit from a Muslim.” (Bukhari and Muslim) He also deserves the Divinely-ordained punishment for the apostate if there is a Muslim state that applies such penalty (hadd).

The Prayer of the one who has nullified his faith is also invalid for the same reasons.

However, these aforementioned rules and consequences of nullifying the faith do not apply – as we have said – until the textual proofs has been made clear to the apostate, he has been asked to repent, the misconceptions which have driven him to nullify his faith have been removed, it has been checked that the conditions are applicable and there is absolutely nothing to prevent this specific person from being declared a disbeliever.

The factors preventing a specific Muslim from being declared a disbeliever are many. Among them are: ignorance, falling under compulsion, holding improper interpretation, error and lack of intent.

Furthermore, establishing the proof against him is quite relative. The proof may be established against one person but not against the other. It may be established by one person and not by the other. It is not necessary when the proof has been established against a particular person that it will automatically be established against each and every Muslim in the world.
The basic rule in terms of establishing the proof is that this should be done by someone in authority, such as a scholar qualified to make ijtihad, or an a Muslim ruler, such as a Sharia judge in Islamic societies, and the like.

It is necessary to be highly careful in this respect, for it is less serious for a Muslim to err in judging an apostate to be a Muslim than to err in expelling a Muslim from the fold of Islam.