can I call this people atheist?

Question

If someone uttered words of disbelief or mocked Islam or disbelieved in pillers of faith or disbelieved in Allah and his attributes and powers can I call him a Kafir/disbeliever/atheist?

Answer 1

If they actually have willingly done what you claim then it is permissible to call them kafir/disbeliever as they have committed Kufr and the word falls true on them. It would be in the first case mentioned in the hadith:

أيما امرئ قال لأخيه: يا كافر، فقد باء بها أحدهما، إن كان كما قال، وإلا رجعت عليه

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.

^{— Muslim}

Uttering words of disbelief or mocking does nullify Islam, even if done in jest:

قل أبالله وآياته ورسوله كنتم تستهزئون لا تعتذروا قد كفرتم بعد إيمانكم

Say, "Is it Allah and His verses and His Messenger that you were mocking?" Make no excuse; you have disbelieved after your belief.

^{— Quran 9:65-66}

Answer 2

Assalamualaikum waramatullahi wabarakatuhu

You are not allowed to call someone kafir because only Allah knows the people that believe and the people that disbelieve. Also by calling someone a kafir can also make you too as a kafir, there is an Hadith that one said

Sahih al-Bukhari 6103 Narrated Abu Huraira:

Allah's Messenger (ﷺ) said, "If a man says to his brother, O Kafir (disbeliever)!' Then surely one of them is such (i.e., a Kafir). "

So please my brother don't call someone a kafir

Wasalamu alaikum