Bhagavad Gita Verse 40, Chapter 4

ajnyashchaashraddadhaanashcha samshayaatmaa vinashyati |
naayam lokosti na paro na sukham sanshayaatmanaha || 40 ||
 
One who is ignorant, devoid of faith and who doubts constantly is destroyed. Neither in this world, nor in other worlds is peace attained by one who constantly doubts.
 
ajnyaha : ignorant
cha : and
ashradhaanaha : devoid of faith
cha : and
samshayaatmaa : one who constantly doubts
vinashyati : destroyed
na : not
ayam : this
lokaha : world
asti : is
na : neither
paro : other (worlds)
na : no
sukham : happiness
sanshayaatmanaha : one who constantly doubts
 
Having pointed out the qualifications needed in a seeker to gain knowledge of self-realization, Shri Krishna explains the obstacles, or "disqualifications", that prevent us from accessing that knowledge. There are three main obstacles: ignorance, lack of faith, and constant doubting.
 
Fortunate are those who have had the chance to even come across something called spiritual knowledge, scriptures, Gita and so on. But unless one comes across a taste of the scriptures, one spends their entire life totally ignorant of the eternal reality behind the changing world. So it is this ignorance that becomes the obstacle towards the knowledge of self-realization.
 
Secondly, Shri Krishna says that one who lacks faith will never fully gain this knowledge. As mentioned earlier, we have to possess faith in scriptures, in our teacher, in ourselves and in the truth of the eternal essence until we gain the knowledge of self-realization. Without faith, we will not last in the long and arduous journey.
 
Finally, the person who doubts everything will also find it difficult to gain knowledge. Now, to be sure, skepticism and inquiry is absolutely important. In fact, it is encouraged by spiritual teachers. But constant doubting without taking efforts to resolve those doubts will become a huge roadblock in gaining spiritual knowledge.
 
Shri Krishna also says that constant doubting is not just an obstacle on the spiritual path. It also prevents us from living peacefully in the material world. Each time we board a train or a plane, we implicitly have faith that the driver will take us to our destination safely. If we constantly doubt the capability of the driver, we will never be able to go about our daily business.