Meaning of "Kaafir":
The word Kaafir (or Kafir) is derived from the root verb "kafara", which means to deny God, conceal, reject or disbelieve. Therefore, a Kaafir is one who disbelieves, rejects or denies.
Evidence:
Allah of Islam said: There is no God but Allah, and Muhammad is the messenger of Allah. In A-rabic: La Illaha Il Allah Muhammad Ur Rasool Allah
As we can see, Allah of Islam is denying the existence of God.
JF, tell me who is a Kafir here? Allah and Muhammadans who say THERE IS NO GOD, or Christians who say there is GOD?