What is righteousness? And what is the breastplate of righteousness?

"Stand therefore, having your loins girt about with truth, and having on the breastplate of righteousness;" - Ephesians 6:14 KJV

The breastplate covers the vital organs of the body. The righteousness of God is our breastplate that protects our very life. By His death on the cross, Jesus has made us right with Him, having our sins washed away by baptism in Jesus name and filling us with His righteous holy Spirit, giving us victory over sin and death.

“Righteousness,” according to Thayer’s Lexicon, is “…The condition acceptable to God…The doctrine concerning the way in which man may attain to a state approved of God.”

We find that by the doctrine of salvation through the gospel that we can obtain righteousness.

The gospel declares the death of Jesus on a cross paying the penalty for our sins, his burial, and resurrection. (1Corinthians 15:1-4)

"For he hath made him to be sin for us, who knew no sin; that we might be made the righteousness of God in him." - 2 Corinthians 5:21 KJV

"For as many of you as have been baptized into Christ have put on Christ." - Galatians 3:27 KJV

When we obey the gospel, we repent, are buried with Him in baptism in the name of Jesus for the remission of our sins, and resurrected to a new life in righteousness by the infilling of His Spirit. (Acts 2:38, Romans 6:1-4)

"For if by one man's offence death reigned by one; much more they which receive abundance of grace and of the gift of righteousness shall reign in life by one, Jesus Christ.)" - Romans 5:17 KJV

Another translation: “For the sin of this one man, Adam, caused death to rule over many. But even greater is God’s wonderful grace and his gift of righteousness, for all who receive it will live in triumph over sin and death through this one man, Jesus Christ.” - Romans 5:17 NLT

When we have obeyed the gospel, our old sinful nature is buried. We put on a new nature when we are filled with God’s Spirit, righteous and holy.

"That ye put off concerning the former conversation the old man, which is corrupt according to the deceitful lusts; ... And that ye put on the new man, which after God is created in righteousness and true holiness." - Ephesians 4:22, 24 KJV

Another translation: “throw off your old sinful nature and your former way of life, which is corrupted by lust and deception… Put on your new nature, created to be like God—truly righteous and holy.” – NLT

"But ye are not in the flesh, but in the Spirit, if so be that the Spirit of God dwell in you. Now if any man have not the Spirit of Christ, he is none of his. And if Christ be in you, the body is dead because of sin; but the Spirit is life because of righteousness. But if the Spirit of him that raised up Jesus from the dead dwell in you, he that raised up Christ from the dead shall also quicken your mortal bodies by his Spirit that dwelleth in you." - Romans 8:9-11 KJV

"To wit, that God was in Christ, reconciling the world unto himself, not imputing their trespasses unto them; and hath committed unto us the word of reconciliation. Now then we are ambassadors for Christ, as though God did beseech you by us: we pray you in Christ's stead, be ye reconciled to God. For he hath made him to be sin for us, who knew no sin; that we might be made the righteousness of God in him." - 2 Corinthians 5:19-21 KJV