The breastplate was a central part of the Roman soldiers armor. It provided protection for the torso which contains vital organs like the heart, lungs, etc. If a battling soldier didn't have a breastplate, he would basically be asking for death.

What does righteousness have to do with such an important piece of Roman armor? To answer that, we must look at what exactly righteousness is. 1 Corinthians 15:34 says this, "Awake to righteousness, and do not sin; for some do not have the knowledge of God. I speak this to your shame." To be righteous is not to sin and do only what is right in God's eyes. Psalm 119:172 "My tongue shall speak of Your word, for all your commandments are righteousness." God's commandments are righteous meaning that they are perfect and without sin. In contrast, lawlessness is a sin, and sin is the opposite of righteousness. So to be righteous, we must obey God's laws.

So back to the question, what does righteousness have to do with a breastplate? If soldiers wear breastplates the otherwise fatal attacks are thwarted. If christians wear righteousness, which means following God's 10 commandments, we will be protected from Satan's firey darts. So now you know what righteousness is and why it's so important, but how do you equipt it? The answer is you follow the commandments. Do what God tells you to, take it one step at a time, and then you will be equipt with the breastplate of righteousness so that you can withstand Satan's attacks.