In the armor of God (Ephesians 6), the breastplate is the breastplate of what?

Answer: Righteousness

Ephesians 6:14 describes the breastplate of righteousness, alongside the belt of truth.

“Stand firm then, with the belt of truth buckled around your waist, with the breastplate of righteousness arrayed,”— Ephesians 6:14 (BSB)