Praise be to Allah.
This is a strong warning, which against and stop everyone from describing a Muslim as a disbeliever without proof and before establishing proof against him.
In a Hadith, Hudhayfah ibn al-Yamaan (may Allah be pleased with him) said: The Messenger of Allah (blessings and peace of Allah be upon him) said:
“What I fear for you is a man who recites the Qur’an, then when its beauty begins to appear on him and he seems to be adhering to the path of Islam, he begins to drift away, then he discards it and throws it behind his back, and he goes to his neighbour with a sword in hand, accusing him of shirk.”
I said: O Prophet of Allah, which of them is more deserving of being accused of shirk, the accused or the accuser? He said: “The accuser.”
Narrated by Ibn Hibbaan in his Saheeh, vol. 1, p. 282.
In this narration, it’s clear that some people may rise and started describing those who truly believe in Allah and His Prophet (PBUH) as a disbeliever.
But there is a point to be clear about, that doesn’t say we should not tell people they did wrong or the belief they hold on to is not the right belief.
However, calling them disbeliever is not the first thing to do, we should clear our mind and always want to show them the right way, not to call them a disbeliever.
We must be extremely cautious about describing any Muslim as a disbeliever, for one of the gravest of sins is for a Muslim to describe his Muslim brother as a disbeliever when he is innocent of that.
A similar idea that explains the reason we should be cation and stop describing a Muslim as a disbeliever without proof is the report narrated by al-Bukhaari (6104) and Muslim (60) from Ibn ‘Umar (may Allah be pleased with him), who said:
The Messenger of Allah (blessings and peace of Allah be upon him) said: “Any man who says to his brother, ‘O disbeliever,’ it will apply to one of them. Either it is as he said, otherwise it will come back to him.”
However, as I said earlier on, this warning only applies in the case of one who accuses a Muslim of being a disbeliever when he is innocent of that, and he did so without any proof.
But, if the one who is accused of shirk deserves it, then there is no blame on the accuser because he is describing him as he deserves to be described.
That said, the first thing to do when one sees someone doing an act of disbeliever is to let him understand the clear proof.
The Prophet (blessings and peace of Allah be upon him) said: “Any man who says to his brother, ‘O disbeliever [kaafir],’ it will apply to one of them. Either it is as he said, otherwise it will come back to him”, meaning if the one who is accused of being a disbeliever has done something to deserve to be described as such, then there is no blame on the one who says to him, O disbeliever.
Shaykh Ibn Baaz (may Allah have mercy on him) said:
“Any man who says to his brother, ‘O disbeliever [kaafir],’ it will apply to one of them” means: if he does not deserve that. That applies if he says to him, O disbeliever when he does not deserve that. However, if he does deserve that, then this description is befitting to him.
from Fataawa Noor ‘ala ad-Darb (6/253).
And Allah knows best.