The Five Obstructing Obstacles:
The Blessed Buddha once said:
There are 5 kinds of mental barrenness, that obstruct any
mental growth and any spiritual progress. What are they ?
1: Skeptical doubt about the perfect Awakening of the Buddha
is the first mental barrenness.
2: Skeptical doubt about the absolute Truth of the Dhamma
is the second mental barrenness.
3: Skeptical doubt about the Purity of the Noble Sangha
is the third mental barrenness.
4: Skeptical doubt about the Efficacy of the mental Training
is the fourth mental barrenness.
5: Anger towards one's friends & fellows on the Noble Path
is the fifth mental barrenness.
These 5 kinds of mental barrenness disable all initiative,
enthusiasm, energy, effort, exertion, and endurance...
Therefore do they eliminate any effective training & thereby
also all the advantageous fruit of such mental training...
Review the mind regularly so to recognize these obstructions
and cure them with faith, investigation, and friendliness.

See also:
http://What-Buddha-Said.net/drops/II/Doubt_and_Uncertainty.htm
http://What-Buddha-Said.net/drops/II/How_to_Overcome_Doubt.htm
http://What-Buddha-Said.net/drops/II/Curing_Doubt_and_Uncertainty.htm
The Moderated Speeches by the Buddha. Majjhima Nikāya [I 101]
http://www.pariyatti.com/book.cgi?prod_id=25072X
http://www.accesstoinsight.org/canon/majjhima/index.html