Ephesians 6:14b "... and having put on the breastplate of righteousness..."
The breastplate of righteousness is represented as a breastplate because with righteousness on our chest, God is now able to look at us. He cannot look at sin. So once we take up our cross and make Him our number one priority, God covers us with is holiness. Obviously this doesn't make us perfect at all. And I'm not even sure if that means He can totally look at us then. Or even if He can "look" at us before we are questions. I'd be lying if I said I knew, but having the breastplate of righteousness just portrays the idea that we now have a direct line to God.
Lemme know what you think if you don't agree or have another meaning for the breastplate.