Making light of Salaat is counted among the Greater Sins. It is mentioned in some reports that the one who considers the prayers unimportant is not eligible for intercession.
Thus Imam Ja'far as-Sadiq (a.s.) said:
"Our intercession is not for those who consider the Prayers unimportant."
Imam Ja'far as-Sadiq (a.s.) said:
"Our intercession is not for those who considered the Prayers lightly."
(Behaarul Anwaar vol.3)
It is related from the Holy Prophet (s.a.w.s.) that he said:
"One who considers prayers unimportant is not from me. By Allah, he is not from me. By Allah, he cannot reach near me at the Haudh-e-Kauthar." (the cistern in Paradise)
The above discussion makes it evident that not avoiding sins, committing sins blatantly and then repenting in the hope of intercession is foolishness and a sign of arrogance and negligence.
Excessive Sins Destroy the Faith (Belief)
Whatever has been mentioned in the discussion of intercession states that intercession comes to the rescue when the person concerned has died with correct belief. It happens, sometimes that, due to excessive involvement in sins and delay in repentance, faith in religion is erased from the heart. Subsequently he enters the limit of doubt and reaches the point of denial (Kufr). At this time if he is taken away by death, such a person is similar to the one who had consumed poison in the false hope that the doctor would save his life. But when the doctor arrives, death had already preceded him. What can the doctor do to cure him. In the same way the request of the intercessors is ineffective for the one who has died as an unbeliever.
"So the intercession of intercessors shall not avail them."
(Surah Muddassir 74:48)
In order to prove my statement I propose to quote one verse from the Holy Qur'an and two traditions:
"Then evil was the end of those who did evil, because they rejected the communications of Allah and used to mock them."
(Surah Rome 30:10)
Sins Blacken the Heart
The first tradition is reported from Imam Ja'far as-Sadiq (a.s.): He said,
"There is no man who does not have a white spot in his heart. When he commits a sin a black spot emerges from this spot. If he repents, the blackness goes away but if he sinks in sin and continues to sin the blackness keeps on increasing till it surrounds the whiteness of the heart completely. When the white spot is totally covered by the blackness, the owner of this heart will never revert towards goodness. This tradition is in consonance with the utterance of Allah:
"By no means! But on their hearts is the stain of the (ill) which they do."
(Surah Mutaffefeen 83:14)
The Blackened Heart is Immune to Advice and Counsel
The people with the 'blackened heart' are totally unaffected by any counsel because their sins have caused the darkness to spread all over their heart. The loss of the inner eye prevents them from perceiving truths nor can they recognise the truth when they see it. They cannot accept any exhortation and revert to the path of virtue. It is in this connection that Imam Ja'far as-Sadiq (a.s.) explains:
"A man resolves to perform a sinful action but does not act upon it, but sometimes does commit it; Allah sees him and says: "By My Glory, after this I shall never forgive you."
According to another report, the person (sinner) is kept away from the Mercy of Allah due to his sin. He also does not get the impetus to seek repentance, hence his sin is never forgiven. Allama Majlisi (r.a.) writes in the explanatory note of the above tradition: "The purport of this tradition of Imam Ja'far as-Sadiq (a.s.) is that, the Imam has commanded us to fear all types of sins because every sin seems to be such that it may not qualify being overlooked.
To Be Fearful of the Past Sins
The believing people should always be fearful of the sins committed in the past and should mourn for them since we do not know which of our sins can bring about out destruction. The Imam (a.s.) has not informed us of any sin that we can ignore and remain bereft of the Mercy of Allah. However, it is certain that the sin for which we have not repented would be sure to prevent our salvation. So we must enter the domain of Allah's grace through the door of repentance. We should sincerely repent for those sins, which are forgotten by us and especially seek forgiveness of those sins that we remember. The method of repenting shall be explained later Inshallah.
Intercession Should Neither Raise False Hope, Nor be a Cause to be Arrogant
The preceding discussion has shown that the element of intercession does not become the cause of arrogance and disobedience. Rather, intercession imparts strength to the sinner in a state of despair. It creates in him the habit of repenting with fervour. Consequently, he would be able to reach a high position and achieve nearness to the Lord of the Worlds.
The Fear and Dread of Sins is Still Necessary
It should not be so, that a person is hopeful of intercession and is negligent at the same time. Along with the hope of intercession the fear of Allah is also a must. Because fear is not contrary to intercession. The one who is hopeful of the grace and mercy of the Almighty Lord may also experience fear. Otherwise he may not be able to obtain intercession of his masters [The Holy Prophet (s.a.w.s.) and the Imams (a.s.)] for a very long time. In other words, such a person would be able to receive intercession only after being afflicted with the punishment of the grave for a long period of time. In the meantime the fear, awe, agony, suffering and misery that he is experiencing, may become the cause for him to receive intercession of the purified Ahlul Bayt (a.s.). It may also spur him to be attached to them steadily.
source : GREATER SINS\Shaheede Mehraab Ayatullah-ul-Uzma Al-Haaj Sayed Abdul Husain Dastghaib Shirazi (r.a.)