2 Corinthians 11:3

But I am afraid that, even as the serpent beguiled Eve by his cunning, your minds may be corrupted and led away from the simplicity of (your sincere and) pure devotion to Christ. 

We need to control the thoughts in our minds. We shouldn’t just let the mind wander down futile paths that cause us to lose our peace. When we use the word to speak against the thoughts that come, we stand spiritually according to the word. The devil will steal our peace if we allow him to steal our thoughts.