The general concepts that the heart of the believer must be firm about are the "pillars" of this faith. But one will not be called a believer just by knowing and understanding these pillars but he must come to the level where he submits and implements what is described, in the hadith of Gabriel, as Islam. Iman, in this manner, incorporates Islam.
If iman was simply knowing the facts in one's heart, then its companion would be equal to Satan and Pharaoh [Note: The Satan was the most knowledgeable of his Lord but he was destroyed because of his pride and envy. And Pharaoh, even though he claimed to be the lord, knew that the
lord is Allah and that none has the right to be worshipped but Him. Allah says, "He said: In truth you know that none sent down these portents save the Lord of the heavens and the earth as proofs..." [al-Isra, 102]. - Although they knew the truth, they did not put it in practice by turning their `ibadah to Allah Alone].
In the hadith of Gabriel, the Prophet explained the pillars of this faith in which every human must believe, when he was asked, "What is iman?", he said, "To believe in Allah and His angels and His books and His messengers and the Last Day and predestination of good and evil."
It is a must for every person to know these pillars and to learn them with a correct understanding and to believe in them in the manner that he pious forefathers understood and believed in them, in the same manner that the Companions of the Prophet believed and understood them, as well as their Followers and those who followed on their path. This includes the four imams, Sufyan Al-Thauri, Sufyan ibn Uyaina, Abdullah ibn al-Mubarak and others similar to them, as well as Muhammad ibn Ismail al-Bukhari, Muslim ibn Al-Hajjaj, Shaikh al-Islam ibn Taymiya and al-Hafedh ibn al-Qayyim. And scholars similar to them who followed the same manner of understanding and believing in these pillars.
This is the first obligation upon the responsible human being. There is no difference of opinion on this question among the scholars whose opinions are worth following.Imam Abu-Hanifa said, "The understanding of faith is better than understanding of the science." What he meant by faith is tauheed and what he meant by science is the shariah. He put the understanding of tawheed before the understanding of the shariah.
And Shaikh al-Islam al-Haruwi al-Ansari (d. 481 AH) stated at the beginning of his book, Itiqad ahl al-Sunnah, "The first obligation upon the slave is the knowledge of Allah. This is proven by the hadith of Muadh, when the Prophet said to him, 'You will come to a people from the People of the Book. The first thing that you should call them to is the worship of Allah. If they gain the knowledge of Allah, then tell them that Allah obligates upon them five prayers during the day and night..."
From this premise did the great scholars of Islam precede. Ponder, for example, what Imam Muhammad ibn Ismail al-Bukhari did in his book al-Jami al-Sahih, which is the most authentic book after the Book of Allah; one will see that from his detailed knowledge and understanding of this religion, this great Imam began his book with "The Beginning of Revelation" and then he followed it with chapters on faith, followed by the chapters on knowledge. As if he means, may Allah have mercy on him, to point out that the first obligation upon a human being is faith or Iman and the way to attaining faith is knowledge. And the source of faith and knowledge is revelation. So he began by showing how the revelation occurred and what it was like. Then he followed by mentioning faith and knowledge. This arrangement is no accident; by it he makes some important points.
This is the sum of what we wish to mention and what we wish to raise our voices about. The matter of aqidah is the first priority. Faith and knowledge are the means of attaining it. And the source of knowledge and faith is the Book and Sunnah.