وَإِذْ قُلْنَا لِلْمَلاَئِكَةِ اسْجُدُواْ لآدَمَ فَسَجَدُواْ إِلاَّ إِبْلِيسَ أَبَى وَاسْتَكْبَرَ وَكَانَ مِنَ الْكَافِرِينَ
And behold, We said to the angels: "Bow down to Adam" and they bowed down. Not so Iblis: he refused and was haughty: He was of those who reject Faith. [Quran 2.34]
Notice the use of the word 'kafireen' in the Arabic. This is one of the reasons why one must learn Arabic to understand the full meaning of the Quran, the words of Allah. Kafir is derived from the word meaning 'to cover' and Kafir also means 'disbeliever in God'. But in this case, Iblees (Satan) is fully aware of Gods' existence and power yet he is still described as a kafir! This goes to show that a person can be a kafir even if he believes in a God. In other words, just because someone believes there is a God, that someone can still be a kafir and be (technically) condemned to hell. Therefore, a kafir can be more correctly defined as someone who 'rejects submission to God' even though he is fully aware of His existence, as well as meaning a person who is a true Atheist - one who 'rejects the existence of God'.