"Breastplate" is a weird name for a prayer, but it makes sense. In Ephesians 6:10-18, Paul speaks of the "armor of God." Specifically, he writes, "Therefore put on the full armor of God, so that when the day of evil comes, you may be able to stand your ground, and after you have done everything, to stand. Stand firm then, with the belt of truth buckled around your waist, with the breastplate of righteousness in place, and with your feet fitted with the readiness that comes from the gospel of peace." (Ephesians 6:14-16)

Early christians took Paul's words as allegories for spiritual tools, even to the point that there are vesting prayers for the Priest's vestments that link each item with a part of Paul's armor!

One famous allegory is St. Patrick's Breastplate, and we've used it many times this Pandemic season as an introduction to our evening Complines