The essence of strength and valor is in the heart.
For a person who is strong-hearted does not fear any person or any thing, and is able to perform awesomely valiant acts and to win fierce battles through the strength and fortitude of heart.
He fears nothing and runs right into the heat of battle.
Exactly the same thing is true in the service of G-d. Understand this well.
Rebbe Nosson of Breslov. Kitzur Likutey Moharan I: 249