People lie because they do not know and heed Allah as they should. Those who are aware that Allah sees and hears them at all times, knows their heart and innermost thoughts, and knows all of their thoughts, and who understand this reality, never lie. They strive to avoid incurring Allah's punishment and take refuge in Him, and do their best not to utter a single word that will earn them His displeasure in the Hereafter. If they do utter such a word, through thoughtlessness or error, they correct it immediately and ask Allah to forgive them.
The main characteristic of liars is that they either do not believe in the afterlife or that they doubt their faith and therefore do not think very much about the afterlife. As Allah tells us in the Qur'an, "those who do not believe in Allah's Signs are merely inventing lies..." (Surat an-Nahl, 16:105)
Those who have some degree of belief in Allah fear the recompense that their lies will bring them in the Hereafter and so avoid lying. In several verses, one of which is related below, Allah informs us that the traps of those engaged in evil plotting are broken:
There is only one way for a person to abandon lying and all forms of immorality: heeding Allah, believing in the Hereafter, and fearing the punishment of Hell. Allah tells us about this truth: "… You can only warn those who fear their Lord in the Unseen and perform prayer. Whoever is purified, is purified for himself alone. Allah is your final destination." (Surah Fatir, 35:18)
In addition, people should expose lies on every occasion and avoid any silent complicity with those who lie by pretending to believe them. Rather, they should make the people admit their lies by not remaining silent, for if they do not so, the liars will take pride in the "fact" that he has "deceived" them and will not abandon his bad behavior.
In order to truly abandon lying, people should consider the real state of those to whom they are talking, for lies are told to earn respect or personal advantage. When people understand that everyone is a powerless servant of Allah and that only gaining His good pleasure—as opposed to that of others—is important, there is no longer any reason to lie.