Source:
https://books.google.de/books?id=I2Wz4HEoOgYC&lpg=PA88&hl=de&pg=PP1#v=onepage&q&f=false
The definition of being a Muslim and the definition of being a kafir are depending on the fundamental definition of Islam itself ultimately, which is quite ironic if you think about it. Two things that ought to be the opposite of each other are being defined by the same set of rules (Islam).
However, every religion claims the truth. So, basically, every non-believer could be considered as kafir. But this is not an unique character of Islam. For instance, in the eyes of the majority of the orthodox Jews, we non-Jews (goyim) are kuffar too.
As a Muslim we know, that it all depends on one's religious attitude towards kuffar. If you accept that even non-believers can do good deeds that Allah is demanding from us humans (such as supporting the poor, preserving the nature, exploring the creation of Allah etc.), the concept of kuffar changes dramatically.
Theoretically, a kafir could do more good deeds in his life than a sinful Muslim who harms the society he lives in. A kafir who helps Muslims and a Muslim who hurts other Muslims cannot be treated equally, hence, I choose Ibn Arabi's approach.