When a heart becomes hardened

“But because of your hard and impenitent heart you are storing up wrath for yourself on the day of wrath when God's righteous judgment will be revealed.” (Rom. 2:5 ESV)

The Lord sees and weighs the heart

“… For the Lord sees not as man sees: man looks on the outward appearance, but the Lord looks on the heart.” ( 1 Sam. 16:7 ESV) 

“Every way of a man is right in his own eyes, but the Lord weighs the heart.” (Prov. 21:2 ESV)

A hardened heart darkens understanding

“And Jesus, aware of this, said to them, “Why are you discussing the fact that you have no bread? Do you not yet perceive or understand? Are your hearts hardened? (Mark 8:17 ESV)

“For they did not understand about the loaves, but their hearts were hardened.” (Mark 6:52 ESV)

“They are darkened in their understanding, alienated from the life of God because of the ignorance that is in them, due to their hardness of heart.” (Eph. 4:18 ESV)

Vigilantly keep your heart 

“Keep your heart with all vigilance, for from it flow the springs of life.” (Prov. 4:23 ESV)

Blessed is the one who fears the Lord always, but whoever hardens his heart will fall into calamity.

 God gives a new heart

“And I will give you a new heart, and a new spirit I will put within you. And I will remove the heart of stone from your flesh and give you a heart of flesh.” (Eze. 26:36 ESV)

Therefore…

“But exhort one another every day, as long as it is called “today,” that none of you may be hardened by the deceitfulness of sin.” (Heb. 3:13 ESV)

“Let the words of my mouth and the meditation of my heart be acceptable in your sight, O Lord, my rock and my redeemer.” (Psalm 19:14 ESV)

“You will seek me and find me, when you seek me with all your heart.” (Jer. 29:13 ESV)

 Quotes:

“God is the shaper of your heart. God does not display his work in abstract terms. He prefers the concrete, and this means that at the end of your life one of three things will happen to your heart: it will grow hard, it will be broken, or it will be tender. Nobody escapes.” - Ravi Zacharias

“You must keep all earthy treasures out of your heart, and let Christ be your treasure, and let Him have your heart.” - Charles Spurgeon

“The great and important duty which is incumbent on Christians, is to guard against all appearance of evil; to watch against the first risings in the heart to evil; and to have a guard upon our actions, that they may not be sinful, or so much as seem to be so.” - George Whitefield