Imam Khomeini explains the matter in his famous work of "combat with the self" as following:
Actually, man would not recklessly run wild if he considered it all likely, he need not have certainty, that which is reported in the Noble Qur’an is right, the promises and the threats, and that he should amend his ways and deeds.
If you consider it all likely that ferocious beasts are to be found along the path which might harm you, or that there are armed bandits who might hold you up, you would refrain from taking that path, and you would try to ascertain the correctness or incorrectness of these reports, Imam further explains.
Is it possible for someone to consider it possible that hell exists and that one may remain forever in its fire while at the same time doing wrong?
Can it be said that one who considers God Almighty to be present and watching and Who sees himself to be in the presence of the Lord, and who considers it possible that there should be retribution for his words and deeds, a reckoning and chastisement, and that in this world every word he speaks, every step he takes, every deed he does, is recorded by angels of Allah called ‘Raqib and ‘Atid,’ and they carefully record all his words and deeds, and in such a state, could he fearless of his own doing?
It is painful [to realize] that they do not even consider these truths to be possible. From the manners of some and their way of living it is obvious that they do not even consider the existence of a supernatural world to be possible, since the mere consideration of this possibility keeps man from committing many wrongs.
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