The eminent Muslim scholar, Sheikh Muhammad Al-Hanooti, member of the North American Fiqh Council, states: “In the hadith reported on the authority of `Umar ibn al-Khattab (may Allah be pleased with him), he said: Angel Gabriel (peace be upon him) came to the Prophet (peace and blessings be upon him) in the appearance of a traveler. He sat in front of him like a student seated in front of his teacher. He started asking the Prophet (peace and blessings be upon him): “Tell me what Islam is.” The prophet (peace and blessings be upon him) told him about the five pillars of Islam. Then the “visitor” asked: “Tell me what Iman (belief) is.” The Prophet (peace and blessings be upon him) answered: “Iman is to believe in Allah, his angels, his scriptures, his s, the Last Day and destiny.”
This hadith is an important reference to tell, firstly, what the pillars of Iman are. The Qur’an, in many occurrences, counts the pillars of Iman except the sixth one, the belief in destiny.
The root of Iman is the three-letter pattern “a-m-n” in Arabic (amina). This root means peace, security, assurance and peace of mind. Prophet Ibrahim (peace and blessings be upon him) asked his people in the story of surat Al-An`am, saying, “Who of the two parties is more apt to have ‘amn’?” (Al-An`am: 81) Then he answers, saying, “Those who believe without staining their faith with injustice are the people who are apt to have ‘amn’ and are guided.” (Al-An`am: 82)
The Prophet (peace and blessings be upon him) did not approve judging a person to be a mu’min (believer) because Iman is centered in one’s heart and appearances are only an expression. Every mu’min is a Muslim, but a Muslim is the one who shows Islam, although he could be a non-Muslim (like hypocrites, who are mentioned in the Qur’an to be in the lowest rank of hell on the Day of Judgment).
Iman increases and decreases according to effects that could enhance one’s deen (religiousness) like attachment to the mosque, to the Qur’an, and to good company.”