We are our thoughts. Our thoughts become words. Our words become actions. Our actions become habits. Our habits become our character.
Good thoughts produce good character.
Bad thoughts not only produce bad character, it opens a way for satan to come in and control us. Once he has some control, we lose control. Then we will grow by the work of the Holy Spirit, and we will have the Mind of Christ. And I was with you in weakness, and in fear, and in much trembling. But as it is written, Eye hath not seen, nor ear heard, neither have entered into the heart of man, the things which God hath prepared for them that love him.