The action of the heart must be practiced; the heart of a Muslim who witnesses evil is influenced by that evil. A dark spot is placed in the heart. “Familiarity breeds contempt” and so is with our hearts. With repeated exposure to evil, the heart will acquire more dark spots until it no longer appreciates good and dislikes evil. So a believer, who does not forbid evil in his heart, might turn into an evil doer himself or herself.