We become what we think.  We are shaped by what occupies our mind.  We have to be on guard for our own heart and thoughts.  The books we read, the movies we watch, the songs we sing, the images we embrace all come together to make us for good or ill.  The forces of darkness are not a friend of a pure mind and a holy heart.  Their agenda is not for us.  A pure mind is an ally to a pure heart.  Watching your thoughts guards your heart.

“The eye is the lamp of the body; so then if your eye is clear, your whole body will be full of light. But if your eye is bad, your whole body will be full of darkness. If then the light that is in you is darkness, how great is the darkness!” Matthew 6:22-23 (NASB)