In order to communicate the truth of Divine Unity, God has sent messengers or prophets to human beings, whose weakness of nature makes them ever prone to forget or even willfully to reject Divine Unity under the promptings of Satan.
According to the Qurʾānic teaching, the being who became Satan (Shayṭān or Iblīs) had previously occupied a high station but fell from divine grace by his act of disobedience in refusing to honour Adam when he was ordered to do so. Since then his work has been to beguile human beings into error and sin. Satan is, therefore, the contemporary of humanity, and Satan’s own act of disobedience is construed by the Qurʾān as the sin of pride. Satan’s machinations will cease only on the Last Day.
Judging from the accounts of the Qurʾān, the record of humanity’s acceptance of the prophets’ messages has been far from perfect. The whole universe is replete with signs of God. The human soul itself is viewed as a witness of the unity and grace of God. Islamic Lectures - TheMuslimPath.com
The messengers of God have, throughout history, been calling humanity back to God. Yet not all people have accepted the truth; many of them have rejected it and become disbelievers (kāfir, plural kuffār; literally, “concealing”—i.e., the blessings of God), and, when a person becomes so obdurate, his heart is sealed by God.
Nevertheless, it is always possible for a sinner to repent (tawbah) and redeem himself by a genuine conversion to the truth. There is no point of no return, and God is forever merciful and always willing and ready to pardon. Genuine repentance has the effect of removing all sins and restoring a person to the state of sinlessness with which he started his life.