"When thou goest out to battle against thine enemies, and seest horses, and chariots, and a people more than thou, be not afraid of them: for the Lord thy God is with thee, which brought thee up out of the land of Egypt. And it shall be, when ye are come nigh unto the battle, that the priest shall approach and speak unto the people. And shall say unto them, Hear, O Israel, ye approach this day unto battle against your enemies: let not your hearts faint, fear not, and do not tremble, neither be ye terrified because of them; For the Lord your God is he that goeth with you, to fight for you against your enemies, to save you." - Deuteronomy 20:1-4
Fainthearted people are people who give up easily. When the heart faints, it just gives up. It has to have everything a certain way or it quits. It gets discouraged and depressed quickly. The person gets his feelings hurt easily. Everything bothers him. He is touchy. In his heart he sayas, "I can't do this. It's just too hard."
In Proverbs 24:10 we are told: "If thou faint in the day of adversity, thy strength is small." We can never stand against the enemy if we are fainthearted. And if we would enjoy the power of a positive mind, we cannot be wimpy or a quitter.
All of us have to resist against getting tired and giving up because we are being hassled by the devil. With God's strength, we don't have to faint, no matter what kind of adversity we are facing. The best way to fight the devil, especially in times of challenge and stress, is to just stay calm, to maintain a peaceful, gentle heart. Be constant, be fearless. That is a sign to the devil of his impending destruction.
And we must not be fainthearted with God's correction of our lives. When God is dealing with us, sometimes He has to do it over and over. Molding is never fun, but we will reap if we do not faint.