Matthew 12:34-35

You brood of vipers, how can you speak good things when you are evil? For the mouth speaks out of that which fills the heart. The good man, from (inner) good treasure, brings out good things; and the evil man, from (his inner) evil treasure, brings out evil things. 

If we fill our hearts with the word and the goodness of God, we will speak good things. If we dwell on negativity and offense and are critical of others, our speech will reflect our hearts. Our hearts are apparent to others by the words we speak. We protect our hearts by our relationship with the word and guidance of the Holy Spirit.