Before we start to explain the meaning of conceit, the damage it causes, its characteristics and how this contemptible case is treated, we ought to pave the way for all of that with some citations from the Qur'ān and the traditions of Ahl al-Bayt, peace be with them all.
Suffices to identify the significance of conceit and the calamity this contemptible case brings about, in the view of the Qur'ān, to read verses 103 – 105 of the blessed Chapter "The Cave", Al-Kahf, where Allāh, Praised is He, says,
"Say: 'Shall we tell you of those who lose the most with regard to their deeds, those whose efforts have been wasted in this life, while they thought that they were acquiring good by their deeds? They are those who deny their Lord's Signs and the fact that they shall meet Him (in the hereafter): Their deeds will be in vain, nor shall We grant them any weight on the Day of Judgment" (Qur'ān, 18:103–105).
We derive from these sacred verses many interesting points on which we are not going to elaborate, but we would only like to point out that conceit, according to these verses, becomes a cause for many good endeavors in this life to go to waste; it leads to disbelieving the Signs of Allāh and in meeting Him, a cause for voiding the good deeds; so, conceit spares no good deed whereby salvation is anticipated. For these reasons, the good deeds of the conceited ones will have no weight on the Judgment Day. This suffices as the destruction caused by this case that brings loss.
As regarding conceit according to traditions, the revered Al-Kāfi traces to Ali ibn Suwaid asking the father of Imām al-Hassan (ع) about conceit which ruins good deeds. The Imām (ع) said, "Conceit is of many levels. One of them is that the bad deed of a servant of Allāh is decorated for him, so he sees it as good and he likes it, thinking he is doing something good. Another level is when a servant believes in his Lord, so he thinks he has done Allāh Almighty a favor while Allāh is the one who bestowed His favor on him."
The Imām (ع) is also cited as having said, "One who is absorbed by conceit perishes." Also he is cited as having said, "Someone sins then regrets. He does a good deed whereby he is pleased, so he relaxes his condition. It would have been better for him to maintain his first status rather than enter into that." The Imām (ع) is also quoted as having said, "A scholar came to a worshipper and asked him about his prayers. The worshipper said, 'Should someone like me be asked about his prayers while I have been worshipping Allāh since such-and-such?!'
He asked him about his tears. The man said to him, 'I cry till my tears pout out.' The scholar then said to him, 'Your laughter while being afraid is better than your weeping while you are thus bragging about it. Nothing of the good deed of a braggart is ever raised.'"
The Messenger of Allāh (ص) has said, "Three things are lethal: an obeyed miserliness, an illicit desire that is followed, and one who is proud of himself." He (ص) has also said, "Had you not sinned, I would have feared for you what is greater than sinning: conceit, conceit."
Ibn Mas`ūd has said, "Perishing is brought about in two situations: despondency and conceit. It is when one loses hope for the mercy of Allāh, desponds from salvation, loses hope from self reform."
Ibn Mas`ūd has combined both of these because one's happiness is pawned by his endeavor and serious effort in seeking it. The Almighty has said,
"… man can have nothing but what he strives for" (Qur'ān, 53:39).
Unless one energetically seeks his objective and original goal, he will never attain the sought happiness. Each of these two characteristics, despondency and conceit, plays a role in slowing down an effort towards one's goal, prohibiting one from seeking it as he ought to. As regarding despondency, the morale of the despondent person is not prepared to pursue his objective. One who desponds from reforming himself, so he is not energetic in order to save it, he may do something that is faster in causing his perdition.
One who desponds from reforming himself does not mind committing any wrongdoing; therefore, despondency from the broad mercy of Allāh is considered as one of the greatest sins. As for conceit, since the conceited person thinks he achieved his happiness and won his objective and goal, he, too, stops his good endeavor and pursuit.
In other words, man does not seek something which is already available rather than something which is impossible. Happiness in the eyes of the conceited person is already present, and in the eyes of the despondent person it is impossible. In this regard, we contend ourselves with this much.