The Breastplate of Righteousness guards your heart (remember we are the dwelling place of the Holy Spirit). We belong to Jesus Christ, we have a Body, Soul, and Spirit and when we speak His Word, His Truth to ourselves we are speaking to that inner man that Spirit man.  Every part of us belongs to Him.
He gives us the ability to stand our ground with His Truth wrapped around us and having that Armor of God's Righteousness in us and surrounding us.