“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.”
This is how John Flavel begins his little book, On Keeping the Heart. This is apparently one of the most popular books he wrote (first published sometime in the 1600s), although I somehow only came across it the other week.
(My other quotations from Flavel’s writings here.)