Bhagavad Gita · Chapter 4: Knowledge and the Renouncing of Action

Bhagavad Gita 4.40

ajñaścāśraddadhānaśca saṃśayātmā vinaśyati | nāyaṃ loko'sti na paro na sukhaṃ saṃśayātmanaḥ ||

But the ignorant, the faithless, and the doubting are lost. For one who is always doubting there is neither this world, nor the next, nor happiness.

Modern context

The doubt condemned here is different from questioning. Arjuna questions constantly and is praised for it. The verse is about the person whose whole nature has become doubt, who trusts nothing and no one and so can never commit to anything. Such a person cannot succeed in worldly life, which needs trust in colleagues, family and one's own decisions. Spiritual life is closed to them for the same reason, and they are not even happy. A question looks for an answer and is finished when it finds one. Chronic doubt is not looking for an answer at all.