The Breastplate of Righteousness
Protecting the heart, the place where we dedicate ourselves to serve God
purity of heart and rectitude of life conformity of heart and life to the divine law. True religion.
The inner being, will, desire, goals, emotions, passions, etc. One of the two places God teaches us (revelation).
Sins of the heart are greed, power, pride, selfishness, worship of material things, etc.
Revelation is the making known of divine truth by communication with the heavens, and consists not only of revelation of the plan of salvation to the Lord's prophets, but also a confirmation in the hearts of the believers that the revelation to the prophets is true. It also consists of individual guidance. (Bible Dictionary-revelation)
The heart is where we gain understanding and often found together in the scriptures.
Understanding occurs when the Holy Ghost confirms in our hearts as true what we know in our minds.
“The multitude did hear and do bear record; and their hearts were open and they did understand in their hearts the words which he prayed.” (3 Nephi 19:33)
The Breastplate then is having a broken heart and a contrite spirit (yielding to God’s will), it is communication (revelation) with God.