A believer must keep two things in his heart: a fear from the consequences of his faults, and a hope for the mercy of Allah accompanied by a good opinion of Him. Having a suspicion about Allah or a despair of His mercy is a grave (mortal) sin.
The Holy Prophet Muhammad (S) quotes Allah as saying: “Those who work for My reward should not rely on their works. For, even if they try their very best to worship Me throughout their lives, they fall short of My due worship, seeking honour, bliss and high ranks in My presence. They should be sure of My mercy and hopeful of My favour, having a good opinion of Me. In this case My beneficence, mercy and forgiveness envelop them. For I am the Beneficent, the Merciful, as I am called.”
He (S) also said: “I swear by Allah besides Whom there is no god, that no believer can ever attain the benefits of this world and the world to come except by his good opinion of Allah, his hope, having a good nature and by abstaining from backbiting believers. I swear by Allah besides Whom there is no god, that no repenting believer will be punished except for his suspicion about Allah, being in despair, having an ill nature and backbiting believers. I swear by Allah besides Whom there is no god, that no believing servant has a good opinion of Allah except that He will behave duly towards His believing servant. For Allah is generous and graceful. When His believing servant has a good opinion of Him, He does not fail to do according to it. So have a good opinion of Allah and wish for Him.”
Imam Muhammad al-Baqir (a.s.) said: “A believing servant must retain two lights in his heart: a light of fear and a light of hope, and these lights must be equal.”
Imam Ja’far al-Sadiq (a.s.) said: “A believer cannot be called such unless he is fearful and hopeful; and he cannot be fearful and hopeful unless he acts according to his fear and hope.”
He (a.s.) also said: “You should not hope for Allah’s mercy in a manner that encourages you to commit sins; nor should you fear from Allah’s punishment in a manner that brings you disappointment.”
He (a.s.) also said: “Having a good opinion of Allah means that you must not hope in anyone except Him and you should not fear for anything except for your sins.”