Will atheists go to heaven?
If you ask the Pope the answer maybe yes.
Even if the atheist accept Allah as the only one worthy of worship before death common sense dictates that every soul will go through the process of Judgement before going
to the final destination. It is up to Allah to make that call.
What is the Quranic definition of Kaafir?
I recommend you, sir, download Dr. Sahib's translation of Holy Quran to get the best answer to this question.
[general Arabic term = the one who hides or covers up the truth can be a Kaafir]
And what does Allah mean when he says"only Deen acceptable to him is islam.
In accordance with the teachings of Quran, as demonstrated by the Holy Prophet, is the only way of life acceptable to Allah. Anything else, such as N2I, is deviation from the