“Shirk is the greatest of sins, because Allah has told us that He will not forgive the one who does not repent to Him from it, but in the case of lesser sins the matter is subject to Allah’s Will; if He wills, He will forgive the one who meets Him with that sin unrepented for, and if He wills, He will punish him. This means that we should fear shirk greatly, as it is so serious before Allah. Moreover, the atheist is destined to the Hellfire in the Hereafter. This is mainly because he refuses the most basic belief, which is the Existence and Oneness of Allah.”
The above quotation is excerpted with slight modifications from www.islam-qa.com
Dr. `Abdul-Fattah `Ashoor, Professor of the Exegesis of the Qur’an at Al-Azhar Univ., states the following: “In kufr (disbelief), there is no greater sin and less great one. All those who deny the Existence of Allah or ascribe partners to Him are kuffar or unbelievers. The destiny of those unbelievers and atheists is the Hellfire. Referring to this, Allah Almighty says: “Lo! Allah forgiveth not that a partner should be ascribed unto Him. He forgiveth (all) save that to whom He will. Whoso ascribeth partners to Allah, he hath indeed invented a tremendous sin.” (An-Nisa’ 48)
In another verse, He Almighty says: “Lo! Allah pardoneth not that partners should be ascribed unto him. He pardoneth all save that to whom He will. Whoso ascribeth partners unto Allah hath wandered far astray.” (An-Nisa’ 116)
Those atheists and unbelievers should search for the truth, denounce their kufr, and repent to Allah. I earnestly advise all atheists and unbelievers to ponder over the universe around them and they will surely see many signs indicating Allah’s Existence.