The Messenger informed Mu'adh bin Jabal, when he was going to the land of Yemen, "You are going to a people from the People of the Book. Let the first thing that you call them to be the worship of Allah. If they acknowledge Allah, then inform them that Allah has obligated upon them five prayers during their days and nights." [Al-Bukhari, Muslim]
This hadith is clear. It does not require much of an explanation. The Prophet applied this principle in his practical calling to Islam. He stayed in Makkah for thirteen years to each the people iman and to educate his Companions on this point and to correct the beliefs of the people. That is the pattern upon which the Companions were brought up.
Jundub Ibn Abdullah al-Bajaly said, "We learned iman (faith) and then we learned the Quran and it increased our iman." Abdullah ibn Umar said, "We lived during an instant of time in which one of us would receive faith first before receiving the Quran and when the surahs were revealed we would learn what they permitted and what they prohibited and what they forbade and what they ordered and what should be the stance towards them. But I have seen many men from whom one is given the Quran before iman and he reads it from the opening of the Book to its closing and he does not know what it orders and what it forbids and what should be his stance towards it. He is like someone who is just throwing out dates [i.e., he does not get any benefit from his recital]."
That is the manner in which the Prophet brought up his companions: Iman first and then the Quran. This is similar to what Imam Abu Hanifa pointed out: Understanding in the religion first (i.e. tauheed) and then understanding in the science (i.e. the shariah).
The beliefs must be corrected first, then follows all of the other aspects of the religion.
And Imam Ash-Shafi`i said, "That a servant meets Allaah with every sin except Shirk is better say than meeting Him upon any of the innovated beliefs."
Al-Aqidah linguistically is derived from the term aqada. In Arabic, one states, "Aqada the rope" when the rope is tied firmly. And, "Aqada the sale" or "He settled the sale" when the person ratifies and contracts a sale or agreement. And Allah says in the Quran, "And as for those whom your right hands have made a covenant (Ar., aqadat)" [Al-Nisa 33]. And Allah also says, "But He will take you to task for the oaths which you swear in earnest (Ar., aqadtum) [Al-Maida, 89] which means asserted and adhered to, as proven in the verse, "And break not oaths after the assertion of them" [An-Nahl, 91]. If one says, "Aqadtu such and such," it means his heart is firm upon such and such.
Therefore, al-aqidah or al-itiqad according to the scholars of Islam is: The firm creed that one's heart is fixed upon without any wavering or doubt. It excludes any supposition, doubt or suspicion.
Imam Abu Hanifa called this great subject al-Fiqh al-Akbar ("The Greater Understanding") and the understanding of the religion. He called the science of law (Ar., shariah) the understanding of the science. Many scholars of Islam use the word tauheed for all matters that a person must believe in. This is because the most important of these matters is the basic tauheed that is contained in the phrase, "There is none worthy of worship except Allah."
Tauheed, according to them, may be divided into two categories: tauheed of cognition and affirmation and tauheed of purpose and deeds.
Tauheed of cognition and affirmation is the tauheed of the Oneness of the Creator and the tauheed of His Names and Attributes [i.e. He is Unique in His being the only Creator and the only One with His names and attributes]. Tauheed of purpose and deeds is tauheed of lordship or that none should be worshipped except Allah [i.e., He is the only One worthy of worship].
The scholastic theologians (Ar., kalaamiyoon) - and what will explain to you who the scholastic theologians are - call this great subject "the root of the religion" and they call the law "the branches of the religion". This is their terminology. We also have a dispute with them in this matter but this is not the place to discuss it. All of them give it a name or adjective according to their needs.
But what is the name the Quran gives to this matter?
The Quran gives the grave matter the name iman. Allah says in the Quran, "And thus We inspired in You (Muhammad) a Spirit of Our command. You did not know what the Scripture was, nor what the Faith was. But we have made it a light whereby We guide whom We will of our bondmen..." [Al-Shura, 52].