Imam Khomeini explains the matter in his famous book "an exposition on Forty Hadith" as following:
If this happens, once his eyes are closed on this world and opened to world of barzakh and, thereafter, to the Day of Resurrection, he will see that the persons who committed major sins are better off than him.
He will see that God Almighty has immersed them in the ocean of His grace and mercy ‘for the remorse and penitence that they expressed, or for the sake of the trust and confidence that they had in the grace and compassion of the Almighty; whereas this poor fellow, since he had considered himself as not in need of His grace and had believed himself, in the depth of his being, to be above needing His compassion and mercy, God Almighty has dealt with him rigorously in regard to his account, and as he himself wished his account is being examined in the scales of justice.
God Almighty will show him that he not only did not perform any worship for His sake, but also that his works of devotion and piety only took him away from God and have removed him away from his goal, Imam further explained.
Not only his faith and deeds have been invalid, but they have also served as the cause of eternal damnation and painful chastisement in hell.
God forbid, lest God Almighty judges anybody with the criteria of His justice, because, as mentioned previously, not a single individual, from the first to the last, will find any way of redemption.
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Imam Khomeini warned that human beings have a perilous and long journey ahead