Because it takes prolonged, religious effort to achieve the most valuable possession – wisdom – those without faith in religious practices (karma-yoga) cannot attain it. They are thus bereft of wisdom’s gifts: happiness in this life and the next.
This verse describes three types of faithless people:
Agya – The ignorant
This refers to people who know nothing about religion.
Ashraddadhana – The faithless
This refers to those who know about religion, but conclude that it is useless.
Samshayatmana – The doubtful
This refers to people who know religion and understand its sublime value, yet are unable to follow it due to irrational doubt and hesitancy Arjun, for the moment, most closely resembles the third, the doubtful. He knows his duty, yet he is unable to follow through. Krishna therefore stresses that this position is the worst out of all three. Why? ...
Na sukham samshayatmana – No happiness for the doubtful
None of the faithless can truly attain spiritual bliss or lasting material prosperity. In that sense, they all "perish" (vinashyati). But those who are ignorant about religion and those who write it off as bogus can at least enjoy moments of wanton sense gratification, whereas the hesitator who knows the path of religion, cannot enjoy even this. He is therefore counted as the worst off of the three.