“A good man brings good things out of the good stored up in his heart, and an evil man brings evil things out of the evil stored up in his heart. For the mouth speaks what the heart is full of” (Luke 6:45, NIV).
Whatever we meditate on, and whatever we set our mind to dwell on, is something we will eventually speak. The tongue is powerful, and when we speak something it may come to pass. This can work for good or bad.