A basic lesson of the Christian life is that all sin begins in our thoughts, which the Bible often calls “the heart.”
Jesus said: Mark 7:20-23 What comes out of a person is what defiles them. For it is from within, out of a person’s heart, that evil thoughts come, sexual immorality, theft, murder, adultery, greed, malice, deceit, lewdness, envy, slander, arrogance and folly. All these evils come from inside and defile a person.
No one commits these outward sins without first having committed them in his mind. If we want to grow in godliness, we must win the battle over sin on the thought level.