Take the helmet of salvation and the sword of the Spirit, which is the word of God. - Ephesians 6:17

A Roman soldier’s helmet was made of hardened leather and was strengthened with metal. They were meant to protect the soldier’s head from blows inflicted from the enemy.

In Ephesians 6, Paul says to “take the helmet of salvation.” He was writing to people who had received salvation through trusting in Jesus. The metaphorical helmet of salvation is a spiritual helmet, not to grant a person salvation, (Paul wasn’t saying take the helmet of salvation to be saved) but it was meant to protect the believer’s assurance of salvation.