Keep your heart with all vigilance,
for from it flow the springs of life.—Proverbs 4:23
You work at keeping your conduct in line, and you work at maintaining a good reputation, but you don't work enough on keeping your heart. The problem with this is unless you learn to keep your heart, your conduct and reputation will be of little value and may come crashing down in times of weakness.
The call to keep your heart is a call to work on your life internally, not merely externally. The latter is easy; the former is much harder and more complicated. The religious or moral person will focus on the external and maintain good appearances, but it may have little to nothing to do with the heart. God is first and foremost concerned with your heart, for when you are keeping your heart, the rest of life follows.
To keep your heart means that your focus and work is on maintaining communion with God and pursuing the transformation that only God can accomplish in you. It is not performance-based religion, nor the moral improvement of your life, but the ongoing work of cultivating love for God and hatred for sin. It is the unending effort of guarding ourselves against idols while resting in the promises of the gospel.
To keep your heart is your primary business as a Christian, and it cannot be done with passing interest or any small amount of energy. It requires the consistent use of all the means of grace. You must make the most of worship, Scripture, prayer, and the church gathered in all its forms with an aim at keeping your heart and growing in grace. If you are doing any less than this, you are keeping up appearances but not your heart. And you know that the heart is what God is primarily interested in (Psalm 51:16-17)—hearts that are broken over sin, healed by God's forgiving grace, and consequently filled with love for our Redeemer God.
—Joe Thorn, Note to Self (Crossway, 2011), 97-98.