The worst of all the evils is to deny (the existence of) Allâhu ta’âlâ, i.e. to be an atheist. It is disbelief not to believe in any one of those things that one must believe. It is kufr not to believe in Muhammad 'alaihis-salâm. (Îmân) means to accept by heart all the commandments revealed to Muhammad ‘alaihis-salâm’ by Allâhu ta’âlâ and delivered by him to us, and to state this belief with the tongue. When there is an obstruction to say it openly, it is permissible not to state it with the tongue. To have îmân, saying or using things which Islam prescribes as signs of kufr must be avoided too. It is disbelief to slight the Ahkâm-i islâmiyya, that is, any of the commands or the prohibitions of Islam, to make mockery of the Qur'ân al-kerîm, angels or any of the Prophets, or to deny anything communicated by them. To deny means not to believe, not to affirm after hearing it. To have doubts about things that are necessary to believe would also mean disbelief.

There are three types of disbelief: 1) Disbelief out of ignorance(jahlî), 2) Disbelief out of obstinacy (juhûdî), and 3) Disbelief byjudgement (hukmî).