What is kufr?
Kufr in the Arabic language is to cover or conceal something.In shari'ah terms it is “not believing in Allaah and His Messenger, whether that is accompanied by denial or it is not accompanied by denial but rather doubt, or turning away from faith out of jealousy or arrogance, or because one is following whims and desires that prevent one from following the message. So kufr is the attribute of everyone who rejects something that Allaah has commanded us to believe in, after news of that has reached him, whether he rejects it in his heart without uttering it, or he speaks those words of rejection without believing it in his heart, or he does both; or he does an action which is described in the texts as putting one beyond the pale of faith.”
See Majmoo’ al-Fataawa by Shaykh al-Islam Ibn Taymiyah, 12/335; al-Ihkaam fi Usool al-Ahkaam by Ibn Hazam, 1/45.