Believing in Allah, may He be Exalted, is one of the pillars of faith, and the most important of them.
Allah, may He be Exalted, says:
{The Messenger has believed in what was revealed to him from his Lord, and [so have] the believers. All of them have believed in Allah and His angels and His books and His messengers, [saying], “We make no distinction between any of His messengers.” And they say, “We hear and we obey. [We seek] Your forgiveness, our Lord, and to You is the [final] destination”} [Al-Baqarah 2:285].
And in the hadith of Jibril, it says: He [Jibril] said: Tell me about iman (faith). He [the Prophet (blessings and peace of Allah be upon him)] said: “It is to believe in Allah, His angels, His Books, His Messengers, and the Last Day, and to believe in al-qadar (the divine will and decree), both good and bad.” He said: You have spoken the truth. Narrated by Muslim (8).
Islam is also mentioned in this hadith, in which the Prophet (blessings and peace of Allah be upon him) described testifying to Allah’s oneness (shahadah) as one of its pillars: [Jibril] said: O Muhammad, tell me about Islam. He said: “Islam means to testify that there is no god worthy of worship except Allah and that Muhammad is the Messenger of Allah, to establish regular prayer, to pay zakah, to fast Ramadan and to perform pilgrimage to the House (the Ka`bah), if you have the means.” He said: You have spoken the truth.
When iman (faith) is mentioned alongside Islam, iman refers to what a person believes in his heart, and Islam refers to his outward deeds. Thus what is meant by faith (iman) in Allah is certain belief in the heart that Allah is one, with no partner or associate in His divinity, His Lordship and His names and attributes.
And what is meant by the shahadah (testimony of faith) that is one of the pillars of Islam is uttering this testimony and declaring it.
The outward and the inward are interconnected, so inward faith in the heart cannot be valid without physical actions that confirm it, and outward faith displayed in phsyical actions cannot be valid without faith in the heart on which those actions are based.
If iman (faith) in Allah is mentioned on its own and is not accompanied by any mention of the word Islam, it includes both the outward and inward aspects, and what is meant is belief in the heart and testifying that Allah is one, with no partner or associate. Thus the outward and the inward are both included in this case, as we see in the hadith about the delegation of [the tribe of] `Abd al-Qays and what the Prophet (blessings and peace of Allah be upon him) said to them:
“I will enjoin four things upon you and forbid four things to you:
I enjoin you to believe in Allah, and do you know what belief in Allah means?
It means to testify that there is no god worthy of worship except Allah, establish prayer, give zakah and give one-fifth (khums) of any booty you seize.
And I forbid four things to you…”
Narrated by al-Bukhari (4368) and Muslim (17).
Thus he included the shahadah (testifying) as part of iman, because iman is mentioned on its own here, not in conjunction with Islam, thus it includes both outward, physical actions and inward faith in the heart.
We see something similar in the Prophet’s words: “Faith has seventy-odd – or sixty-odd – branches, the best of which is saying La ilaha ill-Allah (there is no god worthy of worship except Allah), and the least of which is removing something harmful from the road, and modesty is a branch of faith.”
Narrated by al-Bukhari (9), and Muslim (35), from the hadith of Abu Hurayrah.
Here he also described shahadah (testimony) as part of iman (faith).
Ibn Rajab (may Allah have mercy on him) said: When one of the words Islam and iman is mentioned on its own, it includes the other, and when it is mentioned on its own, it includes the meanings indicated by the other word when it is mentioned on its own as well.
If the two words appear together, one of them will indicate some of the meanings that it conveys when used on its own, and the other word will indicate the rest of the meanings.
This was stated clearly by a number of leading scholars. Abu Bakr al-Isma`ili said in his letter to the people of the mountain: Many of Ahl as-Sunnah wal-Jama`ah said that iman includes both words and deeds, and Islam means doing what a person is obliged to do, when both words are mentioned one after the other. Hence it is said that when the words believers and Muslims are mentioned together, one of them has a meaning that is different from the other, but when one of them is mentioned on its own, it includes the meaning of both words together. Al-Khattabi also mentioned this idea in his book Ma`alim as-Sunan, and a number of other scholars followed him in that after he died. The soundness of this idea is indicated by the way in which the Prophet (blessings and peace of Allah be upon him) explained the meaning of iman when he mentioned it on its own in the hadith about the delegation of `Abd al-Qays, and the way in which he explained Islam when mentioned alongside iman in the hadith of Jibril.
And in another hadith he explained Islam in the same way as iman, as in the report narrated in Musnad al-Imam Ahmad from `Amr ibn `Abasah, who said: A man came to the Prophet (blessings and peace of Allah be upon him) and said: O Messenger of Allah, what is Islam? He said: “Surrendering your heart to Allah and letting the Muslims be safe from your tongue (your words) and your hand (your actions).” He said: What part of Islam is best? He said: “Iman (faith).” He said: What is iman? He said: “Believing in Allah, His angels, His Books, His messengers and the resurrection after death.” He said: What part of iman is best? He said: “Hijrah.” He said: What is hijrah? He said: “Shunning bad deeds.” He said: What part of hijrah is best? He said: “Jihad.”
Thus the Prophet (blessings and peace of Allah be upon him) described iman as the best part of Islam, and included physical deeds in that.
This discussion explains the meaning of Islam and iman, but are they one and the same or different?
The scholars of Sunnah and hadith differed regarding that, but the explanation given above resolves the difference. Hence it may be said:
If either Islam or iman is mentioned on its own, there is no difference between them in that case. But if the two words are mentioned together, then there is a difference between them.
The real difference between them is that iman means believing in the heart and affirming it, knowing what you believe in; and Islam means submitting to Allah, humbling oneself before Him and obeying Him, which is to be demonstrated through physical actions. This is din (religion), as Allah calls Islam din in His Book, and in the hadith of Jibril, the Prophet (blessings and peace of Allah be upon him) called islam iman and ihsan “din.” This also indicates that when one of these two words is mentioned on its own, it includes the other, and they only have different meanings when they are mentioned together. In that case, what is meant by iman is belief in the heart, and what is meant by Islam is physical actions.
When Islam or iman is mentioned on its own, it includes the other and indicates the same as the other.
End quote from Jami` al-`Ulum wal-Hikam, 1/106-108.
What you must know is:
Iman involves both words and deeds, and faith in Allah is based on belief in the heart accompanied by words on the tongue – if one is able to speak – and affirming Allah’s oneness by worshipping Allah through prayer and other acts of worship. Whoever does that is a believer, and this is also Islam, which Allah has prescribed for His slaves.
And Allah knows best.