The first and most important pillar of Sanatan-Dharma is truthfulness.
“There is nothing more sinful than untruthfulness. Because of this, mother earth once said, ‘I can bear any heavy thing except a person who is a liar.'” (Shrimad-Bhagavatam 8.20.4)
If one is not truthful, then whatever devotional service the person performs is useless, because mother earth cannot tolerate liars.
No one is free from the sinful reactions of speaking lies.
Breaking promises is untruthfulness
Breaking vows is untruthfulness
Breaking agreements is untruthfulness
Hiding information is untruthfulness
Making things up is untruthfulness
Speaking on a subject matter for which you have not studied is untruthfulness
Giving misguidance is untruthfulness