It is called Îmân to like and to admit sincerely, that is, to believe, all of what Rasûlullah (sall-Allâhu 'alaihi wa sallam) said. Those who believe so are called Mu'min. It is called Kufr not to believe even one piece of what he said, and to doubt if it is good and correct. People who disbelieve so are called Kâfir. Things which Allahu ta'âlâ clearly commands in the Qur'ân al-kerîm are called Fard. Things which He clearly forbids and prohibits by saying "don't" are called Harâm. Things which Allahu ta'âlâ doesn't clearly command but which are acts our Prophet praised or which he habitually did or which he did not prohibit, though seeing them done, are called Sunnat. It is kufr (disbelief) to dislike the sunnat. It is not a sin not to do them, as long as you like them. Those things which are not liked by him, and which also eradicate the blessings of worships are called Makrûh. The things which are neither commanded nor prohibited are called Mubâh. All these commands and prohibitions are called Sharî'at or Af'âl-i mukallafîn or Ahkâm-i Islâmiyya.
Af'âl-i mukallafîn consists of eight aspects: Fard, wâjib, sunnat, mustahab, mubâh, harâm, makrûh, mufsid. Things that are not prohibited, or though prohibited, their prohibition has been abolished through one of the reasons which the Sharî'at accepts as an excuse, a hindrance or a necessity, are called Halâl. All mubahs are halâl. For example, it is halâl to lie in order to reconcile two Muslims. Everything that is halâl may not be mubâh. For example, it is not mubâh, but it is makrûh to go shopping while the adhân is being called. Nevertheless, it is halâl.
It is fard to learn and know the tenets of îmân, and the various fards and harâms. Thirty-three fards are well-known. Four of them are basic; to perform namâz, to fast, to give zakât, and to perform hadj (pilgrimage). These four fards together with îmân are the basis of Islâm. He who has îmân and who worships, that is, he who carries out these four fards is called a Muslim or Muslimân. He who carries out all four of them and abstains from the harâm is a complete Muslim. If one of these is defective or nonexistent, his state of being a Muslim will also be defective. He who does not carry out any of them may be a Mu'min (believer), but he is not a true Muslim. Though such an îmân protects one in this world only, it is difficult to transmigrate to the Hereafter in possession of this kind of îmân. Îmân is like a candle. Ahkâm-i İslâmiyya is like the lantern, the glass globe around the burning candle. The candle and the lantern which contains it represent Islâm and Dîn-i Islâm. The candle without the lantern will go out quickly. Islâm cannot exist without îmân. Therefore, where there is no Islâm, there is no îmân, either.