What you think about in your unguarded moments reflects what your mind dwells upon. What you speak about when your guard is down is a good gauge of what is in your heart (Matt. 12:34). Your mind needs exercise just as your physical body does. To keep your body healthy, you must be careful what you put into it, and you must exercise regularly. To keep your thoughts pure, you must guard what goes into your mind. To exercise your mind, you must contemplate things that are noble and truths that stretch your mind.
Some Christians allow the world to fill their minds with ungodly thinking. Some people seem drawn to concentrate on the negative, choosing to be pessimistic about everything. Some remain satisfied with thinking of the mundane. Others fail to intentionally place Scripture in their thoughts, choosing instead to adopt human reasoning. Others, however, choose to expose their minds to the truths of God–to that which is true, noble, just, pure, lovely, and good.
The things you allow your mind to dwell on will be revealed by the way you live. If you focus on negative things, you will inevitably be a negative person. If you allow unholy thoughts to fill your mind, ungodliness will become common in your life. If you fill your mind with thoughts of Christ, you will become Christlike.
What you fill your mind with is a matter of choice. Choose to concentrate on the magnificent truths of God, and they will create in you a noble character that brings glory to God.