Krishna told Arjun in the Bhagavad Gita, chapter 17 as to what austerity of tongue is. By this austerity the words of a man who practises it are endowed with supernatural powers to prove to be a fact, i.e. whatever he says happens without fail. To speak truth in the manner explained above, to behave truthfully and to practise truth in everyday conduct purifies the inner faculties of man and by the purification of inner faculties he is nearer god. Lying negates that. A truthful person lives a bright life, whereas an untrue existence seeks covers, shade and concealment, thriving usually in the dark.
Truth is evergreen, and ever new.
Truthfulness is a moral virtue which should be very carefully cultivated. A child is loved and adored and is called as an embodiment of God, because by an nature child is truthful, but we teach him to conceal and prevent truth. A liar is never believed. He loses the confidence of his friends. Every one likes a person with whom he can repose confidence and thus depend on him, but we can never depend on a liar.