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

aśhraddhayā hutaṁ dattaṁ tapas taptaṁ kṛitaṁ cha yat asad ity uchyate pārtha na cha tat pretya no iha

Whatsoever is wrought without faith, oblation, gift, austerity, or other deed, "Asat" it is called, O Pârtha; it is nought, here or hereafter.

In plain words: Krishna says that worship, giving, self-discipline or any other act done without faith is called "Asat," which means unreal or worthless. It brings no good in this life or the next.