The Bible records that David, the shepherd boy chose not to wear the armor of a soldier, that King Saul armed him with, when he averred to fight the giant, Goliath (1 Samuel 17:38-39). But David did wear another kind of armor, an armor that was more effective than the physical armor that he chose not to wear, against an armored foe. It was a spiritual armor that He wore.
According to Hebrews 11:1, faith is defined as the substance of things hoped for the evidence of things not seen. In other words, when one wears the goggles of faith, they can see the invisible. When David faced Goliath, Goliath with his natural eyes saw a ruddy young shepherd boy but when David saw Goliath, instead of seeing the giant, He saw the God far greater and bigger than the giant. With fervency and full confidence in the God who promised to be with His people when they went to war (Deuteronomy 20:4), David knew that it not him or his skills, but it would be God who is true to His word, who delivers the giant into his hands. The mocking arrows found no way to penetrate the shield of Faith that David held on strongly to.
When we arm ourselves with the shield of faith, we can the immortal, invisible, and only wise God (1 Timothy 1:17) and He will fight the battles we are engaged in, giving victory to us, and gaining the glory due Him.
Deuteronomy 20:3-4 (KJV)
3 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;
4 For the LORD your God is he that goeth with you, to fight for you against your enemies, to save you
Psalm 91:4-6 (KJV)
4 He shall cover thee with his feathers, and under his wings shalt thou trust: his truth shall be thy shield and buckler.
5 Thou shalt not be afraid for the terror by night; nor for the arrow that flieth by day;
6 Nor for the pestilence that walketh in darkness; nor for the destruction that wasteth at noonday.