Fortitude (or moral courage) is the strength of mind and character needed to do what is right, no matter how adverse the circumstances or how strong the opposition. In Christian tradition it is considered one of the four cardinal virtues. As well as having the courage to act righteously, fortitude can also describe having the courage not to do wrong, for example, a teenager having enough character to resist peer-group pressure to drink alcohol or take illegal drugs.
There are many historical examples of fortitude, including the early Christians who endured great persecution to spread God's word. Many were martyred rather than do what they thought was wrong or sinful.