To keep the heart then, is carefully to preserve it from sin, which disorders it; and maintain that spiritual frame which fits it for a life of communion with God.
This includes in it six particulars:
1 Frequent observation of the frame of the heart. Carnal and formal persons take no heed to this; they cannot be brought to confer with their own hearts: there are some people who have lived forty or fifty years in the world, and have had scarcely one hour’s discourse with their own hearts. It is a hard tiling to bring a man and himself together on such business; but saints know those soliloquies to be very salutary. The heathen could say, “the soul is made wise by sitting still in quietness.” Though bankrupts care not to look into their accounts, yet upright hearts will know whether they go backward or forward. “I commune with mine own heart,” says David. The heart can never be kept until its case be examined and understood.
2 It includes deep humiliation for heart evils and disorders; thus Hezekiah humbled himself for the pride of his heart. Thus the people were ordered to spread forth their hands to God in prayer, realizing the plague of their own hearts. Upon this account many an up-right heart has been laid low before God; ‘O what an heart have I.’ Saints have in their confession pointed at the heart, the pained place: ‘Lord, here is the wound.’ It is with the heart well kept, as it is with the eye; if a small dust get into the eye it will never cease twinkling and watering till it has wept it out: so the upright heart cannot be at rest till it has wept out its troubles and poured out its complaints before the Lord.
3 It includes earnest supplication and instant prayer for purifying and rectifying grace when sin has defiled and disordered the heart. “Cleanse thou me from secret faults.” “Unite my heart to fear thy name.” Saints have always many such petitions 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’s case, they extend their spirits to the utmost, fill their mouths with arguments, weep and make supplication: ‘O for a better heart! O for a heart to love God more; to hate sin more; to walk more evenly with God. Lord! deny not to me such a heart, whatever thou deny me: give me a heart to fear thee, to love and delight in thee, if I beg my bread in desolate places.’ It is observed of an eminent saint, that when he was confessing sin, he would never give over confessing until he had felt some brokenness of heart for that sin; and when praying for any spiritual mercy, would never give over that suit till he had obtained some relish of that mercy.
4 It includes the imposing of strong engagements upon ourselves to walk more carefully with God, and avoid the occasions whereby the heart may be induced to sin. Well advised and deliberate vows are, in some cases, very useful to guard the heart against some special sin. “I have made a covenant with mine eyes,” says Job. By this means holy men have overawed their souls, and preserved themselves from defilement.
5 It includes a constant and holy jealousy over our own hearts. Quick sighted self-jealousy is an excellent preservative from sin. He that will keep his heart, must have the eyes of the soul awake and open upon all the disorderly and tumultuous stirrings of his affections; if the affections break loose, and the passions be stirred, the soul must discover it, and suppress them before they get to a height. ‘O my soul, dost thou well in this? my tumultuous thoughts and passions, where is your commission?’ Happy is the man that thus feareth always. By this fear of the Lord it is that men depart from evil, shake off sloth, and preserve themselves from iniquity. He that will keep his heart must eat and drink with fear, rejoice with fear, and pass the whole time of his sojourning here in fear. All this is little enough to keep the heart from sin.
6 It includes the realizing of God’s presence with us, and setting the Lord always before us. This the people have found a powerful means of keeping their hearts upright, and awing them from sin. When the eye of our faith is fixed upon the eye of God’s omniscience, we dare not let out our thoughts and affections to vanity. Holy Job durst not suffer his heart to yield to an impure, vain thought, and what was it that moved him to so great circumspection? He tells us, “Doth not He see my ways, and count all my steps?”
In such particulars as these do gracious souls express the care they have of their hearts. They are careful to prevent the breaking loose of the corruptions in time of temptation; careful to preserve the sweetness and comfort they have got from God in any duty. This is the work, and of all works in religion it is the most difficult, constant, and important work.