And take the helmet of salvation, and the sword of the Spirit, which is the word of God:
The helmet of salvation is another part of the armour of God that Paul discusses in Ephesians 6. In the Roman army, the helmet served as protection for the head. It was a vital piece of the armour, as an attack to the head could result in instant death.
For Christians, to receive salvation is to be delivered from our sins and the harmful consequences that come along with them. Jesus Christ paid the ultimate price by being crucified on the cross so that we might have eternal life if we believe in Him.
The consequence we deserve is death, but Christ stood in for us out of His great love. We can find peace when we acknowledge the sacrifice Christ made for us. While this world is corrupt and filled with sin, our salvation acts like a helmet in the way it protects our minds.