अज्ञश्चाश्रद्दधानश्च संशयात्मा विनश्यति। नायं लोकोऽस्ति न परो न सुखं संशयात्मनः ॥

ajñaśh chāśhraddadhānaśh cha sanśhayātmā vinaśhyati nāyaṁ loko ’sti na paro na sukhaṁ sanśhayātmanaḥ

But the ignorant, faithless, doubting self goeth to destruction; nor this world, nor that beyond, nor happiness, is there for the doubting self.

In plain words: Krishna warns that a person who is ignorant, has no faith, and keeps doubting everything ends up ruined. Someone stuck in doubt finds no happiness in this life or the next.