When we are spiritually taught of God to know something of the desperate wickedness and deceitfulness of our hearts, we are prepared to feel the force of this exhortation: "Keep your heart with all diligence, for out of it are the issues of life."
Our blessed Lord has told us, that out of the heart proceed evil thoughts; from where we learn, that the heart is the fountain of all wickedness. Evil thoughts are the springs of evil actions. Until the fountain be cleansed, all the streams which issue from it must therefore be impure.
The heart undergoes a wonderful change when renewed by the Spirit of grace. But, as man is renewed only in part, it becomes the constant duty and work of every believer to keep his heart with all diligence. Sinless perfection is the glory and blessedness of heaven. Here on earth, the most holy servant of God finds daily need of deep humiliation.
"A person who has had a bath needs only to wash his feet." John 13:10. Daily contracted defilement needs daily washing. All the children of God labor to abound yet more and more in all knowledge and in all goodness. Forgetting the things which are behind, they reach forth unto those things which are before; and eagerly press toward the mark for the prize of the high calling of God in Christ Jesus. Uniting with holy David in sentiment and feeling, they can individually say, "I hate vain thoughts, but your law do I love." "Let the words of my mouth, and the meditation of my heart, be acceptable in your sight, Oh Lord, my strength and my Redeemer."
And is this your prayer, Oh, my soul? Are you laboring to maintain a conscience void of offence both towards God and towards man? Is "the thought of foolishness" distressing to you? Can you with Christian sincerity join in this prayer of the Psalmist "Search me, Oh God, and know my heart, try me, and know my thoughts: and see if there be any wicked way in me, and lead me in the everlasting way?" The Scriptures declare, "as a man thinks in his heart, so is he." This habitual inward state of the thoughts determines his character in the sight of God.
"Lord give me grace carefully to observe my thoughts, and to watch and pray, lest being drawn into temptation through the wiles of the devil and the deceitfulness of my heart, I should grieve your Holy Spirit, by whom your people are sealed unto the day of redemption."
Evil thoughts are not our sins, when, being injected by Satan, our will does not consent unto them, but hates and opposes them: and when we earnestly entreat the Lord to save us by his grace from these fiery darts of the wicked one. But as the difficulty lies in ascertaining whether these evil suggestions spring from Satan, or the corruption of our nature, the safest way is to be humbled on account of them: to betake ourselves to Jesus for deliverance from these spiritual enemies, remembering how kindly he has said, "Come unto me all you that labor and are heavy-laden, and I will give you rest." If, through inattention, our souls lie open to the inroads of our ever watchful foe, then the evil thoughts which he stirs up within us, and which are allowed to lodge in our hearts, become our sin. All wanderings and distractions of mind in our religious exercises, arising from lack of watchfulness and due keeping of the heart are sinful.
Those evil thoughts which are excited by dwelling on forbidden objects, reading immoral books, associating with carnal people, or partaking in worldly amusements calculated to inflame the passions, are most awfully chargeable upon us; and will, if not repented of and atoned for through a believing application to the blood of Jesus, sink our souls into endless perdition. If evil, ever bubbling up in the heart, so soon issues into the various actions of the life; how needful to every true believer is this exhortation of Solomon: "keep your heart with all diligence."
In order that our thoughts may please God, they must be brought into captivity to the obedience of Christ. The word of Christ must dwell in us richly, in all wisdom and spiritual understanding, that out of the abundance of the heart, our mouth may speak to his praise and glory. "Your word," says David, "have I hid in my heart, that I might not sin against you." "Whatever things are true, honest, just, pure, lovely, and of good report, if there be any virtue, and if there be any praise, we must think on these things."
We must carefully watch against the first risings of sin, that through grace, the sprouting evil may be nipped in the bud.
We must be much in the habit of mental prayer, lifting up our heart to God on all occasions in humble, fervent ejaculations: which is what the Apostle recommends when he says, "pray without ceasing." This spirit of prayer, this holy habit of devotion, these sacred breathings of the soul, hinder no business except the evil workings of Satan on the mind. This heavenly frame, this delightful communion with the Father of Spirits, forms the purest source of enjoyment to the Christian pilgrim, while journeying through a valley of tears.
To prevent the intrusion of evil thoughts, we must always take care to be usefully employed; since idleness is the soil in which Satan sows his tares with liberal hand. The best way to keep the heart, is that which Jude prescribes; "Keep yourselves in the love of God." We must meditate often on the nature of Almighty God, his majesty and glory, his truth and justice, his holiness and purity, his grace and mercy. We must also contemplate our own apostasy, vileness, and nothingness. We must think much on the love of Christ in dying for sinners, on his agony and bloody sweat, his cross and passion; and then askó "Can I indulge a sinful thought, and cherish in my mind those dreadful evils, which nothing but the blood of God incarnate could expiate and wash away? Can I sin against such transcendent love?"
We must dwell with delight on the gracious operations of the Holy Spirit, in leading the trembling sinner to Jesus; in enabling him to believe with the heart unto righteousness; and in causing him to love that precious Savior, who is the chief among ten thousand, and altogether lovely. We must be continually looking with an eye of faith to Jesus, as our great example; remembering that "those who do not have the Spirit of Christ living in them are not Christians at all." Romans 8:9. He left us "an example that we should follow his steps;" and has declared, "my sheep hear my voice, and I know them, and they follow me." His whole mediatorial character must be the object of our thoughts, until our souls are changed into his same image, from glory to glory, by the Spirit of the Lord.
In order to the keeping of the heart with all diligence, we must labor to set the Lord always before us. We must feel ourselves surrounded with his omnipresence, to whom the darkness and the light are both alike; who weighs the spirits; who is a discerner of the thoughts and intents of the heart.
Oh! my soul, trifle no longer with your thoughts. The irregular desire, the impure look, the angry purpose, though unseen by man, are all recorded by the Omniscient God; and will be condemned as actual transgressions of his holy law, in that day when the secrets of all hearts shall be revealed.
Hasten then to Jesus for grace to save you, and to keep you. Forever renounce all hope of saving yourself by any merit of your own. If "the thought of foolishness is sin," where is the man that lives and sins not?
"Blessed Savior! in you alone have I righteousness and strength. Put forth your mighty power. Deliver me from the assaults of Satan, and the workings of an evil heart.
Enable me to watch and pray, to wrestle and fight, to labor and strive in your promised strength, until conflict shall end in victory; weariness in rest; and mourning in eternal songs of joy."
  With guilt oppressed, bowed down with sin,
Beneath its load I groan;
Give me, dear Lord, a heart of flesh,
Remove this heart of stone.
  A burdened sinner, lo! I come,
An heir of death and hell;
Oh! seal my pardon with your blood,
And all my fears dispel.
  Nor peace, nor rest, my soul can find,
Until your dear cross I see;
Until there in humble faith I cry,
My Jesus died for me.
  Oh! give this realizing faith,
This soul supporting view;
Until old things be forever past,
And all within be new.