One of the gravest sins committed with tongue is lying and it is a sin that can lead to other sins as well.
In order to understand different aspects of every action, we should find the motivation behind it. The Quran points to the motivation behind lying in Verse 105 of Surah An-Nahl: “None forge lies except those who disbelieve the verses of Allah those, they are the liars.”
If one trusts God, His power and knowledge, and His promises, he would never tell lies to gain wealth or positions or other worldly assets. He would not fear poverty or losing prestige or position and would never tell lies to keep them.
An important point about lying is that it is not just a sin but also an action that sets the ground for other sins.
Imam Hassan Askari (AS) said that all the evils have been locked in a room and its key is lying.
When sinners feel threatened and fear disgrace, they start lying to cover their sins. In other words, telling lies allows them to commit different kinds of sins without fearing that their sins may be revealed.
A truthful person, however, avoids sins because he knows he cannot tell lies to hide them.
God in different verses of the Quran has warned people against telling lies, stressing that liars will not reach happiness:
“Say: ‘Those who forge falsehoods against Allah shall not prosper.’” (Verse 69 of Surah Yunus)