Our mind is a battlefield. Many times we have a thinking problem: worry, inferiority, condemnation, thinking as the world thinks, having worldly goals and ambitions. 2 Corinthians 10:5 says that we are to take captive every thought and make it obedient to Christ.
We must put a security guard at the door of our minds and check every thought coming in so that the thoughts can be conformed to the word of God and the will of God. I must meditate on what the word says about myself, the world, God, and God's will.