A faithful man who is dedicated to transcendental knowledge and who subdues his senses is eligible to achieve such knowledge, and having achieved it he quickly attains the supreme spiritual peace. But ignorant and faithless persons who doubt the revealed scriptures do not attain God consciousness; they fall down. For the doubting soul there is no happiness neither in this world nor in the next. (Bhagavad-gita 4.39-40)
Those who are not faithful in this devotional service cannot attain Me, O conqueror of enemies. Therefore they return to the path of birth and death in this material world. (Bhagavad-gita 9.3)
Anything done as sacrifice, charity or penance without faith in the Supreme, O son of Prtha, is impermanent. It is called asat and is useless both in this life and the next. (Bhagavad-gita 17.28)
And I declare that he who studies this sacred conversation of ours worships Me by his intelligence. And one who listens with faith and without envy becomes free from sinful reactions and attains to the auspicious planets where the pious dwell. (Bhagavad-gita 18.70-71)
One who follows the instruction of the Bhagavad-gita, as it is imparted by the Lord, the Personality of Godhead Himself, becomes free from all doubts by the grace of transcendental knowledge.
Therefore the doubts which have arisen in your heart out of ignorance should be slashed by the weapon of knowledge. Armed with yoga, O Bharata, stand and fight. (Bhagavad-gita 4.41-42)