Chazal compare telling a lie to avodah zarah. The Yalkut Shimoni (Toldos 115) states, "Whoever changes his words [and tells an untruth], it is like he worshipped avodah zarah."
Rebbe Simchah Bunim of Peshischa zy”a said that the severity of telling a lie is alluded to in the Torah from the words (Shemos 23:7) תרחק שקר מדבר, "Distance yourself from falsehood..." This is the only aveirah that the Torah emphasizes that we must distance ourselves from transgressing it.
Rebbe Pinchas of Koritz zy”a (Imrei Pinchas 6:78) toiled his entire life to acquire the trait of emes. He would say, “We should consider telling an untruth to be as severe as arayos (adultery, etc.) When Klal Yisrael will be so cautious with saying the truth, Moshiach will come and redeem us.”
What is the connection between telling the truth and the coming of Moshiach?
Rebbe Refael of Barshid zt'l (a leading student of Rebbe Pinchas of Koritz) explains that Moshiach will come when we do teshuvah, and we need to tell the truth to succeed to do teshuvah. Otherwise, we become confused, and cannot tell the difference between a mitzvah and an aveirah.
This is the portion of those who tell lies. Their perception of truth and reality becomes confused until they don't know what is true and what is false, what is good and what is evil. Reb Refael of Barshid explains it on a deeper level. He says that malachim tell lies to the people who lie. The malachim tell them that a mitzvah is an aveirah and that an aveirah is a mitzvah. And when one doesn't know, he won't be able to do teshuvah. Moshiach will come when we do teshuvah, and we will only succeed in repenting our wrong ways when we know which deeds need rectification. This becomes known when we are cautious to tell the truth.
The Imrei Emes of Gur zy”a would say: Everyone should choose one mitzvas aseh [one positive command] and one lav [one prohibition from the Torah] to keep to perfection with mesirus nefesh [under all circumstances].
The Imrei Emes said that he chose the mitzvah of kibud av v’eim [honoring parents] for his mitzvas asei, to excel in, and keeping away from speaking falsehoods for his lo saaseh [prohibitions]. He explained that he chose specifically these two because they are the hardest to keep.
The Shlah Hakadosh (Shaar HaOsiyos 4) writes, “Here, in Yerushalayim, may it speedily be rebuilt, there was a very great chassid from the Sephardic community who would come to me often. For all the money in the world, he would never say anything that wasn’t one hundred percent true. He told me how his father trained him and his brother in this trait. When they were young and would come to their father complaining [about something the other one did], the father would say to them: ‘Whoever tells the truth and admits when he is guilty, I will forgive him, if he promises not to do it again. However, the one who denies and says that he didn’t do anything wrong, and later I discover that he lied, I will punish him severely.' And our father would do just that. For the child who admitted he did something wrong, our father would give him some coins to reward him for telling the truth. And he severely punished the one who lied. In this manner, he raised his children with the trait of emes. This is as it states: (Mishlei 23:23) קנה אמת, 'Purchase truth.' With money and presents, he trained his children to tell the truth until the truth became their second nature. Similarly, all wise people should devise strategies on how to raise their children with good middos and to do good deeds.”
