In the Japuji Sahib, Guru Nanak Dev ji explained what is 'Truth'
'Aadi Such, Jugaadi such, hai bhi Such, Nanak hosi bhi Such'
That, which was before the existence of the worlds and at the appearance of the worlds.
That which is now and 'Nanak' says will be there after the disappearance of the worlds, is TRUTH.
Then a question came up ....... what is Truthful living?
'Kiv Sachiaara hoyiae, kiv koodhe tutay paal?"
How can we become truthful? How can we live a life of 'Truthfulness'?
Guru Nanak Sahib gives a very simple answer in one word.........
"Hukam razaayi chalnaa Nanak likyeaa naal" (Japuji Sahib)
"Surrender to the will of God."
Live according to God's will and all other virtues will follow.
What a simple and marvelous solution to all our problems.