Stand therefore, having fastened on the belt of truth, and having put on the breastplate of righteousness (Ephesians 6:14)

Another piece of God's armor is the breastplate of righteousness. Let us examine what this means. 

Paul again uses an aorist participle with an imperatival sense. He is saying that you have the breastplate of righteousness, but you also must act righteously. There are two things involved here. The first is that we have been given the breastplate of righteousness through faith. This breastplate consists of the imputed righteousness of Christ. Secondly, anyone that has the imputed righteousness of Christ demonstrates this by living righteously. So again, you have been given this piece of armor and now you must use it. 
What do we mean by the “imputed righteousness of Christ?” let’s take a look at 2 Corinthians 5:21.

"For our sake he made him to be sin who knew no sin, so that in him we might become the righteousness of God."

In this text Paul is saying that Christ was in some sense made to be sin, even though he never sinned and we are in some sense the righteousness of God. How is this possible? Christ could not be made to be sin in the sense that he actually committed sins or was sinful. No he was our perfect savior. This means that the only way he could be made to be sin is through imputation. Our sins were transferred to his account. That is what the word ‘impute’ means. It means to credit to one’s account. Christ was made sin in the sense that all our sins were credited to his account. But how have we become the righteousness of God? We are sinful and God is perfectly holy. How could we be the righteousness of God? Through the imputation of Christ’s righteousness. Jesus lived a perfectly holy life. He always obeyed God’s law. He earned a perfect righteous standing in God’s court. Now that perfect righteous standing is credited to our account. God imputes the righteousness of Christ to us so that we become the righteousness of God. This is the breastplate of righteousness.

How do we use this in spiritual battle? One of the schemes of Satan is to accuse us of sin. He brings to mind how bad we truly are and he tries to get us to think that God cannot possibly love us. In other words he beats us down with our sins. It is easy to fall prey to this because it is true that we are sinners. It is true that we have much sin in our lives. However, it is not true that we stand guilty before God. It is not true that God will condemn us for our sins. It is not true that God does not love us. In fact he loves us very much. The reason he can love us is because our sins were credited to Christ’s account and his righteousness was credited to our account. On the cross, Christ did away with the legal guilt of all our sins. Through the imputation of his righteousness, God sees us as perfectly fulfilling the law. He sees us as being holy.

This imputed righteousness is never divorced from actual righteousness. Everyone that has this imputed righteousness will produce some level of actual righteousness. This actual righteousness also protects us from the schemes of the devil. One of the schemes of the devil is to try and discredit our character so that we lose our moral authority. He does this in two ways. First, by falsely accusing us through other people. If we have a reputation of living good lives, it is less likely that anyone will believe these false accusations, unless they have an axe to grind against you. This in turn brings credibility to the Gospel we preach. Second, he tries to discredit our moral authority by trying to get us to fall. If he succeeds, he tells us that we have no right to stand against sin. This in turn makes us hesitant to speak up.  

Through faith alone in Christ we have the breastplate of righteoussness. We are to draw our comfort from this, yet we are also to reflect this breastplate in the way we live our lives.