Assalaamu alaykum. I have seen your links, but my question remains: if you consider somebody who ascribes himself to Islam as a disbeliever but you do not show it outwardly for any reason (for example: shyness, amicable atmosphere at that moment, you like that person, you want him to be pleased with you, and so on). What is the ruling thereof? Is that haram or disbelief? May Allah reward you.
All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger.
Our advice to you is to disregard such satanic whisperings; were they to take control of one’s heart, they would lead him to his own destruction, disturb his life, and spoil his religiosity.
As for takfeer (declaring a person or a group to be disbelievers), there is no doubt that it is prohibited to accuse a Muslim of disbelief with no right. There are many religious texts warning against such an act. For instance, the Prophet said, "If a man says to his brother, 'O disbeliever!' Then surely one of them is such (i.e., a disbeliever). Either the addressed is truly a disbeliever or (if he is not) the accusation rebounds upon the accuser." [Al-Bukhari and Muslim]
Therefore, it is incumbent on you to stop preoccupying yourself with accusing those who are outwardly Muslims of disbelief. You should know that saying or committing acts of disbelief does not necessarily entail declaring the doer a non-Muslim. In fact, declaring a specified individual a non-Muslim entails eliminating the impediments of takfeer such as ignorance, misunderstanding, doubts, and the like. It should be stressed that takfeer lies within the jurisdiction of sharee'ah courts or eligible scholars; commoners are not entitled to accuse any Muslim of disbelief.
However, takfeer of a Muslim with no right does not necessarily entail declaring the accuser a disbeliever as agreed upon among the Muslim scholars. The above-mentioned hadeeth is meant to serve as stern warning against such a grave act; it does not mean that when someone unjustly accuses his Muslim brother of disbelief, he (the accuser) is to be declared a non-Muslim automatically and is considered to be out of the fold of Islam.
You asked 'what is the ruling thereof? Is it haram or disbelief?' If you are referring to not informing the person whom you accused of disbelief of that, then this would be quite odd.
In general, you should know that secretly accusing a Muslim of disbelief and concealing it from him is not considered an act of disbelief nor is it prohibited in any way.
Allaah knows best.
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