Imam explains the matter through his famous work of "Combat with the self" as following:
If one follows the desires of the self and from the day he becomes acquainted with himself pays no attention to anything other than the dark wilderness and never thinks that it is possible that aside from this polluted dark world that there exists another place and station, then he will have sunk into the veil of darkness and have become an instance of,
“But he clung to the earth and followed his low desire” (Q 7:176).
With such a heart polluted by sin that he has been covered by the curtain of darkness, and with such a gloomy spirit that due to the effect numerous sins he has become far from God, the Exalted, that worship of desire and seeking after the world have blinded the intellect and eyes of truth, then he cannot be released from the veils of darkness, let alone to tear the veils of light and detach himself from all but Allah, Imam further explained.
The strongest sort of belief he might have would be not to deny the position of the saints [awliya] of God, and not to consider as myths the worlds of the barzakh, the sirat, the resurrection, the accounting, the book, heaven, and hell.
Due to the effects of sins and the attachment of the heart to this world, one comes to gradually deny these truths, to deny the positions of the saints [awliya] of God, positions which are mentioned in not more than a few lines of prayers and intimate devotions.
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Request to God for vision and brightness, Imam Khomeini explained