Proverbs 4:20-21 (NKJV)
Give attention to my words; incline your ear to my sayings. Do not let them depart from your eyes; keep them in the mindset of your heart.
Proverbs 4:23 (HCSB)
Guard your heart above all else, for it is the source of life.
Proverbs 23:7 (KJV)
For as he thinketh in his heart, so is he.
D. Martyn Lloyd-Jones, Studies in the Sermon on the Mount
“As a man thinks in his heart, so is he.” You can always tell what a man’s philosophy of life is by the way in which he lives and by the way he reacts to the things that are happening round about him. That is why a time of crisis always sifts people. We always betray where we stand by what we say.
Proverbs 23:19, 26
Listen, my son, and be wise, and keep your heart on the right path...give me your heart and let your eyes keep to my ways...
Psalm 57:7, 108:1
My heart is fixed (steadfast, confident), O God
Psalm 19:14
May the meditation of my heart be pleasing in your sight, O Lord.
Psalm 86:11
Teach me your way, O Lord, and I will walk in your truth;
give me an undivided heart, that I may fear your name.
Jeremiah 17:9-10
The heart is deceitful above all things and beyond cure. Who can understand it?
I the Lord search the heart and examine the mind...
Matthew 15:18-19
The things that come out of the mouth come from the heart, and these make a man unclean. For out of the heart come evil thoughts, murder, adultery, sexual immorality, theft, false testimony, slander.
1 Corinthians 4:5, Hebrews 4:13
(When) the Lord comes, He will bring to light what is hidden in darkness and will expose the motives of men's hearts.
A.W. Tozer The Set of the Sail, “The Blessedness of the Fixed Heart”
https://books.google.com/books?id=DngYAgAAQBAJ&pg=PT54&lpg
From Genesis 6:5 we may properly gather that sin has its seat deep within the mind where it pollutes the emotions (desires), the intellect (imaginations) and the will (purposes). These taken together constitute what the Bible and popular theology call the heart.
Richard Baxter A Christian Directory "Thirty Temptations whereby the Devil hindereth Men's Conversion, with proper Remedies against them"
O what a heart we have to watch! It is a lazy heart that will loiter when it should follow. It is a foolish heart that will let him go while it plays with every toy in the way. It is a cowardly heart that will draw back in danger when it should follow our heavenly general. It is a treacherous heart that will give us the slip and deceive us when we are the surest of it. It is short-sighted that even when it follows Christ as guide, it is barely kept from missing the bridge and falling into the gulf of misery.
John Owen
Temptation and Sin
Labour to know what kind of a spirit you are of, what foothold Satan has in your heart, where corruptions are strong, and where grace is weak, and what strongholds lust has in your natural constitution. Get acquainted with your heart. If it is deep, search it out. If it is dark, inquire into it. Though it give other names to all its faults than those they deserve, do not believe it!
Searching Our Hearts in Perilous Times
https://www.monergism.com/thethreshold/sdg/owen/SearchingOurHeartsinPerilous%20-%20John%20Owen.pdf
Thomas Manton A Practical Exposition of the Lord's Prayer
There are so many circuits, wiles, turnings in the heart of man, that we are not competent judges of what is wrought in us; therefore it is usually ascribed to the Spirit to be the searcher of the heart. (Psalm 139:23 - "Search me, O God, and know my heart")
George Swinnock The Christian Man's Calling
The fear of God should stand at the door of our heart to examine all that goes in, lest the traitor, sin, should steal in.
Samuel Lee
Those with a sincere heart busy themselves with heart-work. They seek to mortify sin, quicken grace, and to observe and resist temptations. The more a saint converses with his own heart, the more he understands his spiritual needs and feels his spiritual joys.
John Gibbon “How May We be So Spiritual, as to Check Sin in the First Risings of It?”
Do not be a stranger to yourself. Unlock your bosom, and ransack every corner of your heart. Make a diligent search. Feel the pulse of your soul. Don't let any region of your mind be undiscovered. Draw the curtain of your heart open, and show it God's majesty and ask it if it realizes that He is looking on.
Ezekiel Hopkins "On Glorifying God in His Attributes"
God is present with thee wheresoever thou art, and as much within thee as thy soul is in thy body. He is not a God afar off; but he is near unto thee, even in thy heart, and in the very centre of thy being.
(He) lays his ear to our very hearts; and hears the voice of our thoughts more distinctly, than we can hear the voice of one another's words.
Wheresoever you are, imprint this consideration chiefly upon your hearts: “Now I am with God: he is present with me, in the city, in the field, in the room, in the congregation, in mv closet, in all my ways and converse in the world. And, what! can I be vain, and frothy, and light, when I am before so great and glorious a majesty?”
John Flavel
Keeping the Heart (See below)
The heart of man is his worst part before it be regenerate, and the best afterwards: 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 Touchstone of Sincerity
No man can say what he is, whether his grace be true or false, until they be tried, and examined by those things, which are to them as fire is to gold. That such sufferings as these will discover the falseness and rottenness of men’s hearts, cannot be doubted; if you consider, that this is the fire designed by God for this very use and purpose, to separate the gold from the dross.
The Mystery of Providence
There is so much corruption remaining in good men that they would certainly plunge themselves under much more guilt if providence did not take greater care for them than they do for themselves. Even though they take great care in keeping themselves and daily watch their hearts and ways, yet such is the deceitfulness of sin, that if providence did not lay road blocks in the way, sin would entangle and defile them more frequently than it does. We much thankfully own God's providence, and not impute our escape as a mere accident, or the result of our own watchfulness and wisdom. On the other hand, do not tempt providence by trusting it alone.
A.W. Pink "Heart Work"
https://gracegems.org/Pink/heartwork.htm
Thomas Aquinas
Give me, O Lord, a steadfast heart, which no unworthy affection may drag downwards; give me an unconquered heart, which no tribulation can wear out; give me an upright heart, which no unworthy purpose may tempt aside.