Proverbs 4:23-24

Watch over your heart with all diligence, for from it flow the springs of life. Put away from you a deceitful (lying, misleading) mouth, and put devious lips far from you. 

The heart is the control center of thoughts and actions. The heart holds what we feel, think, and believe. We speak about what we hold inside. We need to guard our mouths and protect our hearts from the world’s influences, so we do not speak about worldly influences or damage our hearts.