2 Corinthians 10:5

We are destroying sophisticated arguments and every exalted and proud thing that sets itself up against the (true) knowledge of God, and we are taking every thought and purpose captive to the obedience of Christ. 

Doubt, unbelief, fear, all begin with a thought! What we are thinking is what we believe. What we believe we manifest in our speaking and then our actions. If we never TAKE a THOUGHT and meditate on it, we will not destroy our Faith and open a door to the enemy. When the enemy whispers in our ear negative thoughts we are to rebuke him in the Name of Jesus.