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.
The helmet is part of an armor that protects the head literally and figuratively the mind. The Bible says that He [God] will keep in perfect peace, whose mind is stayed [fixed] on Him [God] (Isaiah 26:3). The Psalmist (David) avers that the Lord is my light and my Salvation, whom shall I fear? (Psalm 27:1). Those who believe in the Lord Jesus Christ, are assured of Salvation and they can be in perfect peace without any fear. They are assured of eternal life and even death cannot drive fear into their hearts and mind. David recognized that he had no reason to fear the giant Goliath, because he knew that the Lord was his Salvation. He in fact states that boldly to the giant that it will be the Lord who would deliver the giant into his hands; in other words, it will be the Lord who will save him. He had armed himself with the helmet of Salvation and there was nothing for him to fear.
Do you/I have perfect peace as did David? Are you/I armed with the helmet of Salvation that drives out fear?
Psalm 27:1 (KJV)
1 The LORD is my light and my salvation; whom shall I fear? the LORD is the strength of my life; of whom shall I be afraid?