Why does the Quran not explain in detail such an important subject as the Greater Sins?

A Difficult Problem and its Solution
Some people raise the following objections regarding the subject of Greater Sins: -
1. Why does the Quran not explain in detail such an important subject as the Greater Sins?

2. Why are there conflicting reports from Masomeen (a.s.) with regard to the Greater Sins? Some traditions mention that the Greater Sins are five, some give the figure as seven, nine, twenty one and thirty one. One of the traditions reported from Ibne Abbas says that the total number of Greater Sins are seven hundred and seven. Let us now consider these objections so that appropriate replies can be provided. 

Reply to the First Objection

It is Allah's grace that He has not mentioned the total number of Greater Sins in the Holy Quran. There is hidden wisdom in this. Let us suppose that the fixed number of Greater Sins had been given in the Quran. Then the people would have abstained only from these sins. They would have freely committed other sins thinking that they are not doing anything against the Holy Book. In this way they would, be emboldened to perform all other types of misdeeds. Thus Allah has prohibited his creatures to avoid even the lesser sins. This is because if one becomes unmindful of the lesser sins, he will slowly be attracted to the Greater Sins too. 

To Persist in Lesser Sins is Equivalent to Committing a Greater Sin

It is an established fact that if one repeatedly commits a lesser sin he eventually becomes liable for a Greater punishment. Insha-Allah we shall explain in detail the lesser sins in the chapter of "Persistence in Lesser Sins."

By not specifying the number of Greater Sins the Almighty Allah has bestowed a favour upon the people. Had such a number been fixed, the people would have the tendency to abstain mainly from these sins. They would be inclined to regard other sins lightly and indulge in them, underestimating their seriousness. Such indulgence over a period of time makes an individual bold enough to perform other types of misdeeds. It is for this reason that Allah has commanded His creatures to avoid all types of sins, because, being unmindful of minor sins will gradually attract them towards Greater ones. It is indeed Allah's Grace that by encouraging us to abstain from all types of sins, He keeps us safe from the most dreadful ones.

Another important and subtle point that should be stressed is the fact that considering a minor sin to be minor is itself a Greater sin. No sin is to be considered insignificant and trivial. This point shall also be explained in the future. 

- Greater Sins Vol, 1