The heart of man is his worst part before it be regenerate, and the best afterwards: it is the seat of principles, and the fountain of actions. The eye of God is, and the eye of the Christian ought to be, principally fixed upon it.

The greatest difficulty in conversion is to win the heart to God; and the greatest difficulty after conversion is to keep the heart with God.

—John Flavel
The Whole Works of the Reverend John Flavel, 5:423.