Imaan (Faith) & Islam (Self-Submission)
Becoming a Muslim does make you a Mumin (believer) however there are certain guidelines that a Muslim need to adhere to so that their faith does not decrease. In simple terms they need to live with care so that they are not just Muslims by name. Some Bedouins came to the Prophet ﷺ to accept Islam and proudly declared that they now have faith, to which the following verse of the Quran was revealed: (Al Quran 49:14) THE BEDOUIN say, ‘We have attained to faith.’ Say [to them, O Muhammad]: ‘You have not [yet] attained to faith; you should [rather] say, 'We have [outwardly] surrendered' - for [true] faith has not yet entered your hearts. But if you [truly] pay heed to God and His Messenger, He will not let the least of your deeds go to waste: for, behold, God is much-forgiving, a dispenser of grace.” A Muslim is the one who submits his will to the will of Allah i.e. accepts Islam and Mumin is the one who has attained to faith as a result of that submission. In order to become a Mumin one has to first become a Muslim. In order to understand the difference between Muslim and Mumin we need to understand importance of Islam (self-submission) and Imaan (faith).
Islam (self-submission) and Imaan (faith) complement each other. As a sheikh once said that, it is a circle within a circle. The circle of Imaan lies within the circle of Islam. Therefore, one has to enter the circle of Islam first in order to enter that of Imaan. It also holds the position of a heart within a body. The body symbolizes as Islam but the heart inside the body is symbolized as Imaan. If there was no heart the body would be lifeless just as without Imaan Islam would be meaningless. A prophetic hadith teaches that: “there is a piece of flesh inside the body which if it is good then the whole body is good, but if it is corrupt then the whole body will suffer. That piece is the heart.” The functioning of the heart plays a primary role in sustaining the life of our body and so does Imaan in sustaining one’s Islam. If Imaan is strong then our deen (way of life) will be strong and if that is weak then it will be difficult to remain steadfast in Islam.
The emotional attachment towards anything personal in one’s life is dependent upon it being closer to one’s heart. The Imaan begins to enter the heart once we decide to submit our will to the will of Allah i.e. accept Islam and begin following His guidance. The circle of Islam is to give shahadah i.e. testify that la ilaha ilAllah Muhammadur rasool Allah (There is no god but Allah and Muhammad is His Messenger of Allah), to pay zakat (obligatory charity), pray five times salah (prayers), fast the month (thirty days) of ramadaan and perform hajj and umrah if one has the financial ability. These are called five pillars of Islam. In order to taste the sweetness of Imaan an individual has to accept the five pillars and follow them sincerely. The following incident explains how the journey of a Muslim begins with acceptance of Islam with Imaan eventually entering the heart.
A man came to the Prophet ﷺ and chose to accept Islam on the condition that he should be allowed to pray only two prayers instead of compulsory five, to which the Prophet ﷺ allowed him. He accepted Islam by verbal shahadah, became a Muslim and left. Amazed the companions inquired the reason for allowing him to shorten the number of prayers to which the Prophet ﷺ replied “let him begin with two, once he tastes the sweetness of Imaan (faith) he will want to do five.” The verses of Al Quran 49:14 were revealed as a response to arrogant manner of the bedouins, to correct them that they need to first humble themselves and make the intention of obeying the rules and regulations of Islam before Imaan begins to gradually enter their hearts. The obedience is a condition for faith, as said in the above 49:14 “If you obey God and His messenger, He will not put any of your works to waste.” Rules and regulations are given by Allah ﷻ and His Messenger ﷺ and so they needed to obey them first. There is a saying of one of the earlier jurists of Islam named Abu Ubayd Al Qasim ibn Salaam who when inquired about Imaan said “Imaan is to have sincere devotion to God with the heart, to testify this with the tongue, and to act on it with the body.”
Therefore, a man may verbally accept Islam and become a Muslim but in order to become a Mumin he has to become a Muslim who wholeheartedly obeys Allah ﷻ and His Messenger ﷺ and practices Islam with sincerity. One very important thing to mention here is that we Muslims cannot judge any one’s faith, as it is an internal matter within one’s heart. We can only identify a Muslim through their outward actions and not their Imaan, as Imaan is something internal and hidden and the knowledge of unseen only rests with Allah ﷻ.
For more info on what is Imaan and Islam please read the above topic: What is Imaan and Islam?
 Al Quran 49:14 – Asad Translation.
 Sahih Muslim 1599 a
 ‘There is no god but Allah’ is called shahadah (declaration of faith) that Muslims often utter as a sort of zikr (reminder) in their prayers. This declaration is first of the five pillars of Islam.
 770–838 CE
 Heard in one of the khutbah (lectures) in masjid.