Proverbs 4:23
Keep thy heart with all diligence; for out of it are the issues of life.
Your heart determines your life. This may be the most important proverb. The favor or grief in your life depends on ruling and training your heart. If you direct and instruct your heart with godly inputs, there is no limit to your potential success before God and men.
Every sin starts in your heart, and your character and speech reflect your heart. You can only pretend to be different than your heart for a short time, for it will quickly regain control and dictate your actions. Others know your heart by your words and choices.