Importance of Iman
The best and most pure of all deeds in the sight of Allah is Iman. Abu Tharr (may Allah be pleased with him) narrated that he asked the Prophet (peace be upon him): "O Messenger of Allah, which deed is the best?" He replied, "Having Iman in Allah and fighting in His cause (Jihad)" (Narrated by Muslim).
Iman is the path that leads to guidance and happiness in this world and the Hereafter. Allah Almighty says: "So whoever Allah wants to guide - He expands his breast to [contain] Islam" (Al-An"âm: 125).
Iman leads the Mu"min (believer) away from sinning. Allah Almighty says: "Indeed, those who fear Allah - when an impulse touches them from Satan, they remember [Him] and at once they have insight" (Al-A"râf: 201).
Iman is a condition for any deed to be accepted by Allah. Allah says: "And it was already revealed to you and to those before you that if you should associate [anything] with Allah, your work would surely become worthless and you would surely be among the losers" (Az-Zumar: 65).
Sincere Iman makes the deed blessed and makes Du"aa (supplications) be accepted by Allah.