In describing Goliath's armor, the Scripture tells us that he had "greaves of brass upon his legs, and a target of brass between his shoulders." This would seem to be to protect his back. He had also "one bearing a shield" to go before him. (1 Sam. 17:6, 7.) The Christian, on the other hand, has no greaves for his legs, for he is to be "praying always"; he has no armor for his back, for he is to always face the enemy, and never run away. He varies his own shield of faith, for faith held by another is useless for him (Eph. 6:11-18).
Satan, our great adversary, needs the greaves, for he does not pray; he needs armor for his back, for when the Christian resists him he flees (Jas. 4:7). He has no shield of faith of his own, and a shield by another does not save him from the stone out of David's sling.