Ephesians 6:14 says “Stand firm then… with the breastplate of righteousness in place.”
I don’t think it is a coincidence that Paul uses the example of armor in his letter to the Ephesians. Early Christians would have been very familiar with Roman soldiers in full armor. The image of a Roman guard would come right to mind when Paul says “the breastplate.” According to romanmilitary.net, the Romans did not always use a very large breastplate, but in fact a 20cm square piece of armor that they called “the heart guard.” It is likely that Paul and the Ephesians knew that “the breastplate” was synonymous with “the heart guard.” Paul was telling the church at Ephesus to guard their hearts! Perhaps he was describing the more common piece of armor that we picture that covers the whole chest of the soldier, these breastplates would have kept the soldiers up right with good posture, enhancing their intimidation factor and their ability to pay better attention in battle. Either way, Paul was trying to reach the heart of the church, not just give them a description of a man in armor.
Romans 3:22 explains that righteousness is given through Jesus to those that believe. Our righteousness is not our own. We cannot take ownership of it. We are only made righteous through Jesus and His work in us. Just like Paul tells us to put on the full Armor of God, Job says that he puts righteousness on like clothes. (Job 29:14)
We must also be aware that being right, does not always mean being righteous. When the pharisees would attempt to debate Jesus, His responses were always full of love and grace. His objective was never to prove that He was right for the sake of being right. (He didn’t need to, He is God after all.) Jesus wanted to see heart change in people and in His righteousness He knew it would do no good to argue.
It is not in our nature as humans to do what is right. Every fiber of our being is to do what is pleasing to ourselves not what is pleasing to God. However, when we are saved, Jesus’s perfect righteousness is what gives us grace to be imperfect people. The Holy Spirit helps guide us in our decision making and protect us from being corrupted by the world just like the breastplate of a soldier keeps him from being penetrated by a sword.