From Why Did God Give Us Emotions?
Christians sometimes spend a great deal of time pondering whether some emotions, such as love, are good and others, such as anger, are bad. It is similar to wondering whether your hands are good or bad. Emotions, like every other aspect of your being, were originally intended to help you enjoy and serve God. However, just as the actions of your hands can be pleasing or abhorrent to God, your emotions can also serve good or evil. What we need to focus on is the source of the emotion or action.
When the Pharisees saw that Jesus’ disciples were not performing a ritual washing before eating, they questioned Him about it. After pointing out how the Pharisees had become consumed with ritual and failed to do what God commanded, Jesus called the crowd to him and said, “Listen and understand. What goes into a man’s mouth does not make him ‘unclean,’ but what comes out of his mouth, that is what makes him ‘unclean’” (Matthew 15:10–11). When Peter questioned him further on this point Jesus explained that “the things that come out of the mouth come from the heart, and these make a man ‘unclean.’ For out of the heart come evil thoughts, murder, adultery, sexual immorality, theft, false testimony, slander. These are what make a man ‘unclean’” (18-20). Keep in mind that when the New Testament refers to the heart, it is not just looking at it as the center of emotions as modern English does. Instead, the New Testament uses the heart to represent your entire inner being: your mind, desires, emotions, spirit, and soul.
The point is that the source of evil lies in a mind and heart tainted with sin. The actions you take and the emotions you experience are just the outer expression of what is in your heart and mind. Yielding to the Holy Spirit transforms your inner being into what God intended it to be. As this occurs, your emotions, desires, and thoughts are transformed so they function more closely to what God intends.