The breastplate covers the torso of a soldier which contains vital organs such as the heart, lungs and others. Injury to these parts could be very fatal and a soldier without a breastplate could be asking for instantaneous death. The Bible says, “Keep your heart with all diligence, for out of it springs the issues of life” (Proverbs 4:23). The true self of man is assessed by the heart; a heart before God is the man before him (1 Samuel 16:7). The worldly or fleshly man has the inclinations of his heart made of acts of the flesh; wickedness, adultery, sexual immorality, hatred, jealousy etc. (Galatians 5:19, Genesis 6:5). Those who demonstrate any of the acts of the flesh mentioned in the preceding verse, do not stand right (unrighteous) before God.
The Christian, however, is one who is saved from the world and its desires and has their spirits regenerated (born again). This was attained when Christ died for our sins. The Bible clearly states that this righteousness one has attained is not made up of the good deeds you do as such none of us is righteous (Romans 3:10, Ephesians 2:8-9). The breastplate of righteousness is entirely the righteousness of Jesus which He gives us freely when we accept Him as our Saviour (2 Corinthians 5:21). We should therefore continually stand right before our creator with our heart always transformed by the imputed righteousness that contains the Holy Spirit who produces in us good fruit; love, joy, peace, patience, kindness, goodness, faithfulness etc.