Keep thy heart with all diligence; for out of it are the issues of life. (Proverbs 4:23)

The heart of man is his worst part before it is regenerated, and the best part afterward. It is the seat of principles, and the fountain of actions. 
The greatest difficulty in conversion is to win the heart to God. The greatest difficulty after conversion is to keep the heart with God. Direction and help in this great work are the scope of the text wherein we have:
• An exhortation: Keep thy heart with all diligence.
• The reason or motive enforcing it: For out of it are the issues of life.

In the exhortation we'll consider:
• The matter of the duty.
• The manner of performing it.

For out of it [the heart] are the issues of life. In other words, the heart is the source of all vital operations; it is the origin of both good and evil, just as the spring in a watch that sets all the wheels in motion. The heart is the treasury, the hand and tongue are but the shops. The heart contrives, and the members execute: for of the abundance of the heart his mouth speaketh (Luke 6:45).

About the Author
John Flavel (c.1628-1691) was born into an England wracked by political, social, and religious upheaval. Two civil wars and unstable leadership framed the political landscape. Economic hardships and a resurgence of plague further distressed the nation. The church, too, was in turmoil. Flavel, a pastor of one of the many independent churches persecuted by the government, was forced from his church in Dartmouth. In secret and under stress, he continued preaching, writing, and shepherding his flock. He suffered the death of three of his four wives and at least one child. He continued preaching until his sudden death in 1691.