[Keeping the heart] includes earnest supplications and instant prayer for heart-purifying and rectifying grace, when sin has defiled and disordered it. So Psalm 19:12: “Cleanse thou me from secret faults”; and Psalm 86:11: “Unite my heart to fear Thy name.” Saints have always many such petitions pending before the throne of God’s grace. This is the thing which is most pleaded by them with God. When they are praying for outward mercies, perhaps their spirits may be more remiss; but when it comes to the heart case, then they extend their spirits to the utmost, fill their mouths with arguments, weep, and make supplication: “Oh, for a better heart! Oh, for a heart to love God more. Oh, for a heart to hate sin more and to walk more evenly with God. Lord, deny not to me such a heart whatever Thou deniest me! Give me a heart to fear Thee, love and delight in Thee, even if I beg my bread in desolate places.”
John Flavel, Keeping the Heart (SDG, 1998), 7.