Imam Sadiq (peace be upon him): Abu Amr al-Zubayri said: I said to Imam Sadiq (peace be upon him): "O scholar, which of the deeds is the best in the sight of Allah?" He said: "That deed without which Allah accepts nothing else." I said: "What is that deed?" He said: "Faith in Allah, the One, which is the highest, the noblest and the most exalted thing." I said: "Is faith only speech, or must speech and action be together?" The Imam (peace be upon him) said: "Faith is entirely action, and verbal expression is a part of faith that Allah has explained in His Book — its light is clear and its proof is established; the Book bears witness to it and calls one toward it." I said: "Describe faith for me so that I may grasp it well." He said: "Faith has various states and degrees; they are not all at one level, but differ in rank and station. Some faith is complete and at the level of perfection, some is deficient, and some is intermediate." I said: "So faith can be at the level of perfection, and can be deficient, and can also increase?" He said: "Yes." I said: "How is that?" The Imam (peace be upon him) said: "Allah the Exalted made faith obligatory upon the limbs of the children of Adam and distributed it among the organs, assigning to each a fixed share. He assigned each of the duties of man to one of the limbs, and each organ has a specific duty that another organ is not charged with. Whoever guards his organs and limbs and fulfills the duty of each will meet his Lord with complete faith and will enter Paradise, and whoever betrays in any of them or refuses the commands of Allah will step into His presence with deficient faith." I said: "I have understood the deficiency of faith, but tell me — from where does its increase come?" He said: "Allah the Exalted has said concerning this: 'And whenever a Surah is sent down, some of them say [to others]: "Which of you has this [Surah] increased in faith?" [Tell them]: As for those who believe, it has increased them in faith, and they are joyful [at Allah's grace and mercy]. And as for those in whose hearts is a disease, it has added filth to their filth.' (al-Tawba/124-125) Elsewhere He said: 'We narrate to you their story in truth. They were youths who believed in their Lord, and We increased them in guidance.' (al-Kahf/13) If all were equal and there were no increase or decrease in it, then none of them would have any superiority over another; if there were no increase, all blessings would be equal, all people would be alike, and preference would be nullified. But it is with complete faith that the believers enter Paradise, and with the increase in faith they attain higher ranks, and those whose faith is deficient enter Hellfire."