It is believed that the foundation or the root of Iman is in the heart. This has been supported by a large number of verses from the Qur’an and several Hadith of the Prophet Muhammad (peace be upon him). The following examples are provided to illustrate this point. Allah said, ‘Whoever disbelieved in Allah after his belief, except him who is forced thereto and whose heart is at rest with faith, but such as opens their breasts to disbelief on them is wrath from Allah, and theirs will be a great torment.’ (Surah An-Nahl: 106)

In the above Ayah, Allah has considered the actions of the heart as the distinctive point between disbelief and Iman (Faith). Again, Allah said, ‘O Messenger (Muhammad (peace be upon him))! Let not those who hurry to fall into disbelief grieve you, of whom those who say, “We believe” with their mouths, but their hearts have no faith (Surah Al-Ma’idah: 41).

In the above Ayat, belief is made in the heart. Allah stresses the fact that if He seals one’s heart, he is a great loser.

Al-Numan bin Bashir (radi Allahu anhu) narrated: “I heard Allah’s Messenger (peace be upon him) saying, ‘Both legal and illegal things are evident, but between them there are doubtful (unclear) things, and most of the people have no knowledge about them. So whoever saves himself from these doubtful things, he saves his religion and his honor. And whoever indulges in these doubtful things is like a shepherd who grazes (his animals) near the Hima (private pasture) of someone else, and at any moment he is liable to get in it. O People! Beware! Every king has a Hima, and the Hima of Allah on the earth is His illegal (forbidden) things; beware! There is a piece of flesh in the body; if it becomes good (reformed), the whole body becomes good, but if it gets spoilt, the whole body gets spoilt, and that is the heart”. (Al-Bukhari)

The above Hadeeth illustrates several spiritual issues, and one of them is the significance and the superiority of the heart, which, if it is perfect, the whole body becomes good. Thus, the actions of the heart are very significant, and they include articles of faith that can be categorized as follows:
Belief in Allah
Belief in the Oneness of the Lordship of Allah (Tawheed ar-Rububiyya)
Belief in the Oneness of the Worship of Allah (Tawheed al-Uluhiyya)
Belief in the Names and the Qualities of Allah (Tawheed al-Asma wa-Sifat)
Belief in the Angels
Belief in Allah’s Books/Scriptures
Belief in Allah’s Prophets and Messengers
Belief in Alyaumul-Akhir (The Last Day/The Hereafter)
Belief in Al Qada’ Wai Al-Qadar (Preordainment and Fatalism)
Belief in Resurrection
Belief in the Day of Account (Records and Scales)
Belief in Al-Jinn
Belief in Muhammad (SAW)’s Message (peace and blessings be upon him)
Love of Allah and His Messenger Muhammad (peace and blessings be upon him)
Love of the rightly guided Sahaba (Caliphs and Companions of the Prophet)
Love of the Ansar
Belief in the Shafa’t (Intercession) of the Prophet (peace and blessings be upon him)
Belief in the Jannah (Paradise) and in the Nar (Hellfire)
Belief in the questioning in the grave
Fearing Allah and despairing not of His Mercy.
Total and perfect reliance on Allah