10 Finally, be strong in the Lord and in the strength of his might. 11 Put on the whole armour of God, that you may be able to stand against the schemes of the devil.12 For we do not wrestle against flesh and blood, but against the rulers, against the authorities, against the cosmic powers over this present darkness, against the spiritual forces of evil in the heavenly places. 13 Therefore take up the whole armour of God, that you may be able to withstand in the evil day, and having done all, to stand firm. 14 Stand therefore, having fastened on the belt of truth, and having put on the breastplate of righteousness, 15 and, as shoes for your feet, having put on the readiness given by the gospel of peace. 16 In all circumstances take up the shield of faith, with which you can extinguish all the flaming darts of the evil one;17 and take the helmet of salvation, and the sword of the Spirit, which is the word of God, 18 praying at all times in the Spirit, with all prayer and supplication. To that end, keep alert with all perseverance, making supplication for all the saints."
Ephesians 6:10-18
The next part of the Christian armour is what Paul calls 'the breastplate of righteousness'. The imagery here is significant. The breastplate guards the heart, which is the vital organ of the body. If Satan can get to our heart, the battle is won for him and lost for us. And therefore he tries to get to the very heart of our resistance with the arrows of doubt and confusion. If he can make us doubt our salvation, he will not have much trouble with us, for we will be paralysed as to effective battle. The twofold reference we saw to be possible in relation to the girdle of truth applies just as much here. There is the doctrine of righteousness - by which we mean the glorious reality of justification by faith alone, and there are the moral accompaniments of that mighty doctrine, in terms of integrity of life. Two things may be said about this. The first is that there is no question but that the doctrine of the righteousness of God imputed to us and received by faith alone was the essential heart of the Reformation, and that this mighty doctrine is enshrined in the teaching of the Epistle to the Romans. But it is one thing for Christians to have 'accepted Christ as their Saviour' (hardly, be it noted, the language the New Testament uses), but quite another thing - and this is our second point - for them to understand how this puts them in the right with God. And one finds, in experience, that it is those who are confused and hazy about the nature of what has happened to them in their experience of conversion that become easy prey to the wiles of the enemy, and consequently become unsatisfactory in their Christian experience, and do not grow into maturity; whereas those who are clear and instructed in these matters tend to stand firm and follow on to know the Lord. There is a great need for Christian people to get down to basic study and get a right hold of the heart of the gospel, and its deepest meaning. This is what it is to 'put on the breastplate of righteousness', and we shall look in some detail at this in the Notes that follow.