The heart is the key to walking with God. When the heart is changed and sanctified, the life will be godly and will be pleasing to the Lord. But when a man's heart is not guarded and kept for the Lord, there will be problems. This is why the father who is speaking frankly with his sons, will urge them to do all that is necessary to watch over their hearts.
The word "watch" used here is the Hebrew word, "natsar" and it means to watch over something, to guard and to keep it. This word was used to describe how a person is to maintain something that is entrusted to them. Whether we truly realize it or not, God has given us our hearts. He has given us life and a direction in which it should be going. Once this wonderful gift of salvation and a new heart is ours by God's grace - it is our duty to protect it - and to make sure that we watch over what God has done in our lives. This "watching" is to be done with all diligence. The word used here is "mismar" and it means to have someone put into custody, to guard them. It speaks of things that are in a condition of being watched and controlled. What a fascinating word to use here. We are to watch our hearts and do so with a diligence that would be reminiscent of a prison - or the watch of a military unit at night. This is no casual affair, but is something that is of the utmost seriousness. God is absolutely serious about what we are doing with our hearts.
The reason for this "due diligence" over our hearts is because our hearts are vital to our spiritual lives. The father tells his son that from his heart flow the very springs of life itself. A literal way of translating this passage is to say, "from out of it are the issues of life." William Harvey, in the seventeeth century, discovered the circulation of blood in the human body. This one discovery absolutely revolutionized science and medicine. Over 2500 years before Harvey, the writer of Proverbs, inspired by the Holy Spirit, accurately spoke of how the heart was the very location where springs of life flowed out into the rest of the body. The heart pumping the blood of the body through the arteries - is the very wellspring of life for the entire body. If we watch out for the health of our heart - we are doing what must be done for a healthy body. Kill the heart - and the body will die. In a similar way, the heart is the seat of our spiritual life. Keep our hearts and we will maintain a life of holiness and godliness. Let our hearts go and we will find that the life of God will no longer flow freely through us and in us.
The godly father impresses such things on his son. He makes frank statements to his boys about the need to maintain a vital spiritual life where a heart for God is not just maintained, but is encouraged to flourish. The father who takes this job seriously will be the father who rears sons to be men of God. Dad, are you challenging your sons to maintain hearts for God and the things of God?