Who Are 'Al-Kafirun'

The label of knowing the truth and still denying it is not something I as a human being can assign to anyone. I do not know what is inside someone else's heart. So the Quran using the word kuffar or kafirun is a commentary on the reality of what is inside a heart — and only Allah knows that.

Kufr takes many shapes and sizes. The rejection of truth is what the Quran is concerned with — and rejection of truth can take a thousand forms: atheism, polytheism, the rejection of specific revelations, and more. The Quran did not need to spell out Taoists, Buddhists, agnostics, and atheists as separate categories because the root issue it is addressing is the rejection of truth, whatever form that takes.

There are perhaps one or two places in the Quran where something close to atheism is directly addressed. But mostly, kufr in the Quran is defined by what is in the heart — the covering over, the burying of, the rejection of what one has been shown. And only Allah can determine when that threshold has been crossed in any individual.

So al-kafirun in this surah is addressing those whose hearts are in a state of rejection — which includes atheism, but is not limited to people who "know" and consciously deny. The knowing and conscious denial is a subset that Allah addresses in other places as well.