"...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... Heart-work is hard work indeed." John Flavel
Found in Sanctuary of the Soul by Richard Foster
An expanded quote found here...
Heart-work is hard work indeed. Are you inclined to undertake the business of keeping your heart? Are you resolved upon it? I charge you then, to engage in it earnestly. Away with every cowardly feeling, and make up your mind to encounter difficulties. Draw your armor from the Word of God. Let the Word of Christ dwell in you richly in its commands, its promises, its threatenings; let it be fixed in your understanding, your memory, your conscience, your affections. You must learn to wield the sword of the Spirit (which is the Word of God) familiarly, if you would defend your heart and conquer your enemies.