The lexical definition of iman is to know a person as truthful, to believe him/her, and to be fearless. On the other hand, the lexical definition of Islam is to submit and attain salvation. However, their istilah (technical, that is, a different meaning peculiar to the concerned branch of knowledge) meanings are different.
Iman is to have belief in the six fundamental principles expressed in the Amantu and to accept and love all the commandments and prohibitions revealed by Allahu ta’ala, and also to state this belief by word of mouth.
All the rules of our religion, altogether, are termed iman and Islam. These rules have been abridged and summed up in the Amantu as six principles. A person who believes in the six certain facts in the Amantu is called a Mu’min or a Muslim. Iman and Islam are the same.
If iman were only to have belief and Islam to put into practice, the fundamentals of Islam would have been four, not five. However, of them, what ranks first is to say the Kalima-i shahadat, that is, to have belief. What follow it are the principles pertaining to practice. A person who believes and performs other four essentials is called a Muslim.
The things to be practiced, that is, things to be performed and avoided with the heart and body, are called Islam. The place of iman is heart. Islam, on the other hand, is fulfilled with both the heart and the body. Iman is connected to the heart, while Islam is connected to the triad of the heart, the tongue, and the entire body. Iman and Islam in the heart are the same.
Iman is like a candle. Ahkam-i Islamiyya (commandments and prohibitions of Islam) is like the lantern, the glass globe around the burning candle. The candle and the lantern that contains it represent Islam. Islam cannot exist without iman. Therefore, where there is no Islam, there is no iman, either.
One who believes has submitted to the commandments of Allah; that is, one becomes a Muslim. In brief, every Mu’min is a Muslim and every Muslim is a Mu’min. The knowledge pertaining to belief and practice is termed Islam.