Imam explains the matter in his famous book "an exposition on Forty Hadith" as following:
That is, the reason that the believer is afflicted in this world with tribulation is that God Almighty has made it neither the place of His reward for the believer nor the place of chastisement for the infidel.
This world is the abode of duty and the farm of the Hereafter. It is a place of trade and earning whereas the Hereafter is the abode of reward and punishment, of bounty and damnation.
Those who expect that God Almighty would immediately get hold of one who commits some sin or indecency in this world or perpetrates some injustice or aggression against someone, and cut his hand off and expunge him from the realm of existence, are unaware that their expectation is contrary to this world’s order and opposed to God’s wont and sunnah, Imam further explained.
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Imam Khomeini advised believers to perform worship with tranquility of mind
Here is the place of trial and the zone of the separation of the wretched from the felicitous and the sinful from the obedient. Here is the realm of the manifestation of deeds, not the abode of the emergence of the results of personal deeds and qualities. If occasionally God Almighty does take hold of an oppressor, it may be said that it is due to the Almighty’s mercy for that oppressor (for it stops him from sinning further).
For, when God Almighty leaves the sinful and the tyrants to themselves, His wrath takes the form of istidraj, the gradual seizing.
Hence, God Almighty declares:
(And those who cry lies to Our signs), We will draw them on little by little whence they know not; and I respite them - assuredly My guile is firm. (7: 182183)
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Imam stressed need to acquire nobility of character