And it is not permissible to be lenient in declaring someone a disbeliever or an open sinner, for in that there are two great dangers: the first is fabricating a lie against Allah the Exalted in the ruling

The Shaykh :"And it is not permissible to be lenient in declaring someone a disbeliever or an open sinner, for in that there are two great dangers:
The first is fabricating a lie against Allah the Exalted in the ruling, and against the one being judged in the description with which he is charged" Fabricating a lie against Allah in the ruling means, for example, thinking that this act is disbelief and saying that this man is a disbeliever, when there is no text in the Book or the Sunnah declaring him a disbeliever. What happens here? He has fabricated a lie against Allah, and he has also fabricated a lie against the one he charged, saying that he is a disbeliever or an open sinner.
"The second - the second danger - is falling into what he charged his brother with, if the brother is free from it.
In Sahih Muslim, it is narrated from Abdullah ibn Umar (may Allah be pleased with them both) that the Prophet (peace and blessings be upon him) said:

When a man calls his brother an unbeliever, it returns (at least) to one of them.

إذا كفّر الرجل أخاه فقد باء بها أحدهما

Sahih Muslim 60a

In another narration:

If it were so, as he asserted (then the unbelief of man was confirmed but if it was not true), then it returned to him (to the man who labeled it on his brother Muslim).

إن كان كما قال وإلا رجعت عليه

Sahih Muslim 60b

And in it, from Abu Dharr (may Allah be pleased with him), from the Prophet (peace and blessings be upon him):

And whoever calls a man a disbeliever, or says: enemy of Allah - meaning he is the enemy of Allah - and it is not so, except that it returns to him - meaning it returns -

ومن دعا رجلاً بالكفر أو قال : عدو الله - يعني هو عدو الله - وليس كذلك إلا حار عليه - أي رجع -

" .
This second danger is that if a person says to someone that he is a disbeliever, and he is not a disbeliever, then if he is deserving of that, it is as he said; otherwise, it returns to him. But what does "it returns to him" mean? Does it mean that he becomes a disbeliever because of this, or does it mean that he is tested until he disobeys Allah and becomes a disbeliever? The answer is the second. Because if a man says to someone "O disbeliever," and the one addressed is not a disbeliever, we do not say he has become a disbeliever. And for this reason, it came in the hadith: meaning that it is necessary, even if only for a moment, that that description returns to him, unless he repents; if he repents, Allah will accept his repentance. We return to the text: "

When a man calls his brother an unbeliever, it returns (at least) to one of them.

إذا كفر الرجل أخاه فقد باء بها أحدهما

Sahih Muslim 60a

" Who is "one of them"? Either the one declaring disbelief or the one declared a disbeliever. And in a narration: It also clarifies So beware, beware. But if a person says to another: "You are a disbeliever," not intending to issue a ruling on him, but out of anger, saying "You are a disbeliever," then this is not what is intended by the hadith, because the meaning is "You are a disbeliever by this act," as in the saying of Allah the Exalted: Even though fighting believers, what is its ruling? It is disbelief, as he (peace and blessings be upon him) said: .
"

he who labeled anyone with unbelief

ومن دعا رجلا بالكفر

Sahih Muslim 61

" - And what does "calling him a disbeliever" mean? It means saying "O disbeliever" - or saying "enemy of Allah," meaning he is "

the enemy of Allah, and he was in fact not so, it rebounded on him.

عدو االله وليس كذلك إلا حار عليه

Sahih Muslim 61

" .