On belief and faith.

"As to those who reject faith, it is same to them whether thou warn them or do not warn them; they will not believe (2:6)"

Kafara is the root word denoting rejection of faith, the derivatives--Kufr relates to state of rejection, while Kafir is one who rejects faith wilfully. It is related that Kafara has many shades of meaning apart from rejection of faith, it is a state where Allah [SwT]'s goodness is denied, it could also mean ascribing to Allah [SwT] what could be derogatory, that amounts to blasphemy.

In the realm of Islam however while deliberate rejection of faith is decried, mistaken idea of faith or Allah (SwT)'s attributes coupled with a desire to see the truth is taken to be worthy of guidance. Philosophers versed in the wide embrace of Islam like Allama Iqbal (RA) place Kafara and its derivatives—Kufr and Kafir as a state not consistent with Ishq (love) of Allah (SwT). In a significant couplet he relates:

Agar Ho Ishq Tou Haa Kafur Bi Musalmani

Na Ho Tou Mard-e-Musalman Bi Kafir Va Zindeeq

Allama Iqbal (RA) makes Ishq-e-Illahi [Love of Almighty Allah (SwT)] the denominator of being a Muslim--a state of total submission to the will of Allah (SwT) the object of love, worthy of total attention.

And for those who reject faith not out of mistaken belief, but wilfully it is ordained--warn them or not, they will not believe, as their senses cannot perceive the state of spiritual bliss, their hearts thus stay sealed as related:
'Allah hath set a seal on their hearts and on their hearing. And on their eyes is veil; Great is the penalty they (incur) [2:7]