The heart of man is his worst part before it is regenerated, and the best afterward; 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. Here lies the very force and stress of religion; here is that which makes the way to life a narrow way, and the gate of heaven a strait gate.
...though the expression, Keep thy heart, seems to put it upon us as our work, yet it does not imply a sufficiency in us to do it. We are as able to stop the sun in its course, or to make the rivers run backward, as by our own skill and power to rule and order our hearts. We may as well be our own saviors as our own keepers; and yet Solomon speaks properly enough when he says, Keep thy heart, because the duty is ours, though the power is of God; what power we have depends upon the exciting and assisting strength of Christ. Grace within us is beholden to grace without us. "Without me ye can do nothing" [Jn. 15:5].
(John Flavel, On Keeping the Heart. New York: American Tract Society, n.d., p. 5)
The Works of John Flavel, 6 volumes.