The human heart is the seat of all human emotions. A heart that is Full of evil emotions and thoughts is but a favourite place for Satan. Those hearts that are disbelieving, doubtful and mischief-mongering are, according to the Quran, hearts that have been sealed.
Allah says in His book: `thus do we seal the hearts of transgressors’ (10:75) and:`I will cause terror into the hearts of those who disbelieve’ (8:12).
The Quran makes many such references to the human heart — The Book of Allah proclaims numerous merits of a sound heart, a contended heart Full of unbounded love and deep understanding of God and mankind, His creation.