“So stand ready, … with righteousness as your breastplate”. 

Eph 6:14 GNB 
Today security personnel protect themselves with bullet-proof vests – small waistcoat-like jackets with thick padding around their chests. Previous generations had no such protection. 
But the Roman soldier did. Across his chest, and sometimes reaching round his back as well, was a bronze casing to prevent his body being pierced by an enemy spear or sword. It also had the effect of making the soldier look stronger, chunkier and heavier. 
For Paul, observing his guard – and he was chained to him – this breastplate was the piece of armour that best symbolized “righteousness”. This symbolism was probably suggested to him from an Old Testament reference in Isaiah 59:17 where God himself is said to have “put on righteousness as his breastplate” (NIV). In Paul’s usage, “righteousness” often means “justification” which is “God’s gracious initiative in putting sinners right with himself through Christ … Certainly no spiritual protection is greater than a righteous relationship with God. To have been justified by his grace through simple faith in Christ crucified, to be clothed with a righteousness which is not one’s own but Christ’s , to stand before God not condemned but accepted – this is an essential defence against an accusing conscience and against the slanderous attacks of the evil one, whose Hebrew name (Satan) means ‘adversary’ and whose Greek title (devil) means ‘slanderer’”. (J. Stott, The Message of Ephesians, p278). 
Put a little simpler we might say this means the confidence that comes from knowing that Christ has forgiven and accepted you completely and therefore removed all guilt you may otherwise have had. It includes spiritual maturity, the certainty that you are God’s and Christ is yours. All doubt about knowing Christ is gone. 

PRAYER THOUGHT Lord, increase in me the certainty of your acceptance of me.