An array of actions comes into a person’s mind once they hear these words.
God has inspired within each soul the ability, although limited, to recognize truth from falsehood.
"He inspired it (the soul) to know its sin and its piety." (Quran 91:8)
As long as a person seeks righteousness, they will know when they have done something wrong through their conscience, even though they might find numerous ways to excuse themselves for what they are doing. They would never like anyone to come to know of that thing, for they are ashamed of it; their religion is enriched with shyness, shame and bashfulness. The Prophet said:
"Shame and bashfulness is from the perfection of faith." (Saheeh Al-Bukhari)
Shame is something which can prevent a person from committing evil. The Prophet said:
"If you have no shame, then do as you please." (Saheeh Al-Bukhari)
Shame, the highest level being shameful in front of God from committing sins, is a key factor in avoiding sin, and this standard may also be used to judge whether or not an act is a sin.
These sentiments of conscience and shame are a natural consequence to true belief and faith, and this is what the religion of Islam seeks to create within an individual, an Islamic conscience which guides humans through their lives.
This inner conscience is what tells the state of the heart of the individual, whether it is alive seeking the truth, or dead, filled with the desires of this worldly life. Lack of religiosity and indulging in sin causes a person to lose one’s conscience, and it can no longer be used as a source of guidance.
"…why then did they not believe with humility? But their hearts became hardened, and Satan made fair seeming to them that which they used to do." (Quran 6:43)
"Have they not traversed through the land, and have they heart with which they perceive, or ears with which they hear? Indeed it is not the sight which is blinded, but rather what is blinded is the hearts which are in the breasts." (Quran 22:46)
The heart can be used as a guide, in conjunction with the intellect and revelation, in order to ease the search for the truth. The heart of one who is searching for the truth is indeed one which is alive, for it is this life and yearning which causes them to search for it. This type of person will never find peace at heart in any other religion except the religion which God ordained for humanity, and as long as their yearning for the truth exists, their conscience will continue to bother them until they find the true religion of God. Indeed if the person is sincere, God will guide them to the truth:
"And those who desire Guidance, He (God) will increase them in Guidance, and inspire them with [the way to] piety." (Quran 47:17)