'Breastplate' in the Bible

So stand firm and hold your ground, having tightened the wide band of truth (personal integrity, moral courage) around your waist and having put on the breastplate of righteousness (an upright heart),

But since we [believers] belong to the day, let us be sober, having put on the breastplate of faith and love, and as a helmet, the hope and confident assurance of salvation.