" However, here’s a crucial point to remember about the Roman soldier’s protective shield: It was made of leather, so it would have become hard, stiff, and brittle without proper care. If not correctly maintained, over time the leather would have hardened until, when put under pressure, it cracked and fell to pieces. This is why the end result of a soldier’s failure to care for his shield was death on the battlefield. If the Roman soldier wanted to live a long life, it was imperative for him to pick up that vial and apply the oil to his shield every single day of his military life. Because the "

― Rick Renner , Spiritual Weapons to Defeat the Enemy: Overcoming the Wiles, Devices, and Deception of the Devil