The habit of thinking ill of others is an affliction which makes a person blind and pushes him into the dark pit of ruin. It is this quality which brought about the worship of a dead human being. It is this quality, which by suspending operation of the Divine attributes of creation, mercy, providence, etc., reduces God to a useless entity. It would be no exaggeration to state that it is in consequence of this habit that the greater part, indeed the whole, of hell will be filled. Those who think ill of the commissioned ones of God Almighty scorn His bounties and His grace. (Malfoozat, Vol. 1, pp. 95-96)
The habit of thinking ill is a sore affliction which consumes faith as quickly as blazing fire consumes tinder. God becomes the enemy of him who thinks ill of God's Messengers and stands up to fight him. He entertains such jealousy on behalf of His chosen ones has no equal. When I was attacked in diverse ways, the same jealousy of God was roused on my behalf. (Al Wasiyyat, p. 26, footnote)
I tell you truly that the habit of thinking ill of others is a great affliction which destroys a person's faith, flings him far away from the truth and converts his friends into enemies. In order to acquire the qualities of the righteous, it is necessary that a person should altogether shun the habit of thinking ill of others, and should he happen to fall into that attitude concerning someone else, he should seek forgiveness repeatedly and should supplicate to God Almighty that he may be safeguarded against such sinfulness and the consequences that flow from it. This habit should not be underrated. It is a dangerous disease which destroys a person very quickly. (Malfoozat, Vol. II, p. 107)