Helmets are everywhere.  In the UK you must wear one on a motorcycle, on a building site, and when going to work underground.  Skydivers, cavers, rescue services and the military all know that once you lose control of your head, you lose everything.  The Roman army helmet (galea) which Paul was writing about was modified over time, like military kit anywhere - but the constant features were a metal dome to cover the head, an attached metal neck protection, metal side pieces to cover the jaw and sides of the face, and flanges to cover over the tops of the ears.  The helmet protected the brain, so the soldier could be fully self-controlled; the neck, so that a sword blow would not decapitate; the ears, so that the soldier could hear but a glancing blow would not cut off the ear pinna; the jaw, so that the soldier could continue to feed himself.

Paul's analogy is important for Christians.  Our salvation is the helmet we need to protect our mind and eyes and ears and mouth.  Not only does Christ's death on the cross save us from eternal judgement as we put our trust in Him, it reminds us who we belong to, on which side we are fighting.  The helmet of salvation enables us to be clear-thinking about the spiritual battle.  It also provides us with a hope about the future.  We are saved when we repent and receive Jesus Christ (Romans 10:9-13), we are being saved as we are strengthened to overcome temptation and live holy lives (2 Thessalonians 2:13) but we will be finally saved when we are with our Lord in glory (1 Thessalonians 5:8).

We are not just saved from sin and danger; we are saved to be the 'soldiers and servants' of Jesus - to work with Him to rescue those who have been taken hostage by Satan (2 Timothy 2:26).  That is why we need to be fully functional, self-controlled, 'all together', able to listen to His Word and to feed ourselves with it. The salvation-helmet protects all those vital functions.  As the Roman soldier rejoiced in the coloured plumes attached to his helmet (that he was a part of the Imperial Army), so we should rejoice in the splendour of our salvation - that we have been made children of the living God, through faith in Christ.

It is easy to leave home for work, thinking of ourselves as teachers, retailers, farmers, plumbers, engineers, academics, nurses or production line workers.  That is our job; it's what we get paid for.  But if we left home thinking that we are children of the King, agents of heaven and ambassadors of grace (2 Corinthians 5:20) ... what a difference that would make.  Our salvation-helmet may be polished in church, but it is designed to be worn everywhere where Jesus is not honoured.  In those places, the devil will try to neutralise our presence, keep our mouths zipped up and try to intimidate us lest we should try to invade his territory.  But with the confidence that we are saved for eternity, and therefore safe, we will be able to work with the Lord despite the opposition.  The confidence that Jesus has saved us for the purpose of working with Him to rescue Satan's hostages should focus our thinking.  So, put on the helmet, stand your ground and be glad to represent the Saviour Jesus.