Bhagavad Gita Chapter 17, Verse 28

अश्रद्धया हुतं दत्तं तपस्तप्तं कृतं च यत् | असदित्युच्यते पार्थ न च तत्प्रेत्य नो इह ||

aśraddhayā hutaṁ dattaṁ tapas taptaṁ kṛtaṁ ca yat asad ity ucyate pārtha na ca tat pretya no iha

O son of Pritha, whatever is offered in sacrifice, given, practiced, or done without faith—that is called 'not-SAT' (false), and it is of no benefit either here or after death.

Krishna concludes the teaching on OM TAT SAT by explaining what happens without faith. 'Aśraddhayā hutaṁ dattaṁ tapas taptaṁ kṛtaṁ ca yat'—whatever (yat) is offered in sacrifice (hutam), given (dattam), practiced as austerity (tapas taptam), or done (kṛtam) without faith (aśraddhayā). 'Asad ity ucyate pārtha'—that is called (ucyate) 'not-SAT' (asad iti), O son of Pritha (pārtha). 'Na ca tat pretya no iha'—and that (tat) is of no benefit (na) either here (iha) or after death (pretya). Actions performed without faith are 'asad'—not real, not true, not beneficial. They have no benefit in this life or after death because they're not connected to the Supreme. Understanding this emphasizes the importance of faith and OM TAT SAT in making actions meaningful and beneficial.