The Guru Granth Sahib opens with the Mul Mantar, the root formula that states Sikhism's core theological claims in a single dense line, and the second attribute it gives God, immediately after affirming God's oneness, is Sat Naam, whose Name is Truth. Sat is not presented as one virtue among many so much as a description of ultimate reality itself, and Sikh ethical teaching treats truthful living as the human response called for by that description: to live truthfully is to live in a manner consistent with the nature of what is real.
Sikh teaching pairs Sat with sach achar, truthful conduct, distinguishing between merely speaking the truth and living in a manner fully aligned with it, honest labor, honest dealing with others, and freedom from the self-deception Sikh doctrine associates with haumai, the ego that obscures a person's perception of both themselves and God. The Guru Granth Sahib repeatedly ranks truthful living above ritual observance, a recurring theme across the compositions of multiple Gurus, holding that truthful conduct pursued sincerely outweighs external religious performance pursued without it.
The Japji Sahib, Guru Nanak's opening composition, closes its meditation on how truth is reached with the line that a person becomes true by living truthfully, framing Sat not as a fixed possession or a claim one makes about oneself but as an ongoing practice tested in daily conduct, honest work and honest speech.