Unbelief is an unforgivable ingratitude. The one who denies Him Who brought him into existence from non-existence and endowed him with many kinds of faculties like reason intellect heart memory and insight and inner and outer senses and Who nourishes him with numerous varieties of food and drink has doomed himself to eternal punishment.
As pointed out earlier if all the men in the world came together to create a single fruit or leaf or even a single blade of grass they could not do it. So denial of the One Who created this huge universe and subjugated it to the use of man is the severest and most abominable kind of crime which deserves the most lasting and severe kind of punishment. Again the one who following in the footsteps of Satan who invites him to unbelief and dissipation has submitted to the desires and seduction of his evil-commanding self which in fact was given to him so that he might rise to the highest of the high by refining it closed the doors of his conscience a faculty which innately feels the existence of One God. Creator and Sustainer of beings to innumerable signs of God in the person himself and the universe and the one who extinguished his feelings which long for eternity and which can be satisfied only with eternity has condemned himself to eternal punishment. Also the one who blinded himself to the most manifest signs of the Creator namely The Holy Book and the Messenger Muhammad and other Messengers does not deserve less than eternal punishment.