Know that a full explanation of the grounds on which a person may or may not be branded an Unbeliever would require a long and detailed discussion covering all of the various doctrines and schools of thought along with the proofs and pseudo-proofs adduced by each, as well as the manner in which they departed from the apparent meaning of scripture and the degree to which they rely on figurative interpretation. Several volumes would not be enough to cover all of this. Nor do I have time to explain it all. So, for the time being, content yourself with a piece of advice and a maxim.
As for the Advice, it is that you restrain your tongue, to the best of your ability, from indicting the people who face Mecca (on charges of Unbelief) as long as they say, ‘There is no god but God, Muhammad is the messenger of God,’ without categorically contradicting this. And for them to contradict this categorically is for them to affirm the possibility that the Prophet,* with or without excuse, delivered lies. Indeed, branding people Unbelievers is a serious matter. Remaining silent, on the other hand, entails no liability at all.
As for the Maxim, it is that speculative matters (al-naẓarīyāt) are of two types. One is connected with the fundamental principles of creed, the other with secondary issues. The fundamental principles are acknowledging the existence of God, the prophethood of His Prophet, and the reality of the Last Day. Everything else is secondary.
Know that there should be no branding any person an Unbeliever over any secondary issue whatsoever, as a matter of principle, with one exception: that such a person reject a religious tenet that was learned from the Prophet* and passed down via diffusely congruent channels (tawātur). Even here, however, regarding some matters he may simply be subject to being deemed wrong, as is done with legal issues. Or he may be subject to condemnation for unsanctioned innovation (bid‘a), such as with wrong ideas regarding the Caliphate and the status of the Companions.
*صلى الله عليه وسلم
[Imām Abū Ḥāmid al-Ghazālī, فيصل التفرقة بين الإسلام والزندقة (Fayṣal al-Tafriqa Bayn al-Islām wa al-Zandaqa). English translation: On the Boundaries of Theological Tolerance in Islam by Dr. Sherman Abdul-Hakim Jackson, Karachi: Oxford University Press, 2002, pp. 112-113]