The Heart as Center of Hidden Emotional-Intellectual-Moral Activity. “Man looks at the outward appearance, ” says Samuel, “but the lord looks at the heart” ( 1 Sam 16:7 ). The king’s heart is unsearchable to humankind ( Prov 25:3 ), but the Lord searches all hearts to reward all according to their conduct ( Jer 17:10 ). In the time of judgment God will expose the hidden counsels of the heart ( 1 Cor 4:5 ).
Jesus says that the heart’s secrets are betrayed by the mouth, even as a tree’s fruit discloses its nature ( Matt 12:33-34 ). “A wise man’s heart guides his mouth, ” says Solomon ( Prov 16:23 ). Most important, the mouth confesses what the heart trusts ( Rom 10:9 ; cf.  Deut 30:14 ).
Moderns connect some of the heart’s emotional-intellectual-moral functions with the brain and glands, but its functions are not precisely equivalent for three reasons.
First, moderns do not normally associate the brain/mind with both rational and nonrational activities, yet the ancients did not divorce them ( Psalm 20:4 ).
Second, the heart’s reasoning, as well as its feeling, depends on its moral condition. Jesus said that “from within, out of men’s hearts, come evil thoughts” ( Mark 7:21 ). Because the human heart is deceitful above all things ( Jer 17:9 ) and folly is found up in the heart of a child ( Prov 22:15 ), the Spirit of God must give humans a new heart ( Jer 31:33 ;  Ezek 36:26 ) through faith that purifies it ( Acts 15:9 ; cf.  Eph 3:17 ).
Third, moderns distinguish between the brain’s thoughts and a person’s actions, but the distinction between thought and action is inappropriate for heart. “The word is very near you, ” says Moses to a regenerated Israel, “in your mouth and in your heart” ( Deut 30:14 ).
The Heart’s Emotional Functions. The Lord, who knows our hearts ( Luke 16:15 ), experiences its full range of emotions: for example, its joy ( Deut 28:47 ;  1 Sam 2:1 ;  Prov 15:15 ) and its sorrow ( 1 Sam 1:8 ); its raging ( 2 Kings 6:11 ) and its peace ( Col 3:15 ); its feeling troubled ( John 14:1 ) and its rejoicing ( 1 Sam 2:1 ;  Psalm 104:15 ); its love ( Rom 5:5 ;  1 Peter 1:22 ) and its selfish ambition ( James 3:14 ); its modes of doubts ( Mark 11:23 ) and of fear ( Gen 42:28 ) and its mode of trusting ( Prov 3:5 ); when it rises up in repulsive pride ( Deut 8:14 ) or, as in the case of Jesus, is lowly and humble ( Matt 11:29 ); and when one loses heart ( Heb 12:3 ) or takes heart ( John 16:33 ).
The emotional state of the heart affects the rest of a person: “A happy heart makes the face cheerful, but heartache crushes the spirit” ( Prov 15:13 ); “a cheerful heart is good medicine, but a crushed spirit dries up the bones” ( 17:22 ).
The heart also wishes, desires. The father warns his son against coveting the adulteress’s beauty ( Prov 6:25 ) and against envying sinners in his heart ( Prov 23:17 ). Above all else the heart of a saint seeks God ( Psalms 119:2  Psalms 119:10 ). Believers set it on things above ( Col 3:1 ). This is effected, says Jesus, by putting your treasures in heaven, for “where your treasure is, there your heart will be also” ( Matt 6:21 ). If we look for God with all our heart, Moses promises we will find him ( Deut 4:28-29 ).
The Heart’s Intellectual-Spiritual Functions. The heart thinks ( Matt 9:4 ;  Mark 2:8 ), remembers, reflects, and meditates ( Psalm 77:5-6 ;  Luke 2:19 ). Solomon’s comprehensive knowledge of flora and fauna is described as his breadth of heart ( 1 Kings 4:29 ).
More specifically, as the eyes were meant to see and the ears to hear, the heart is meant to understand, to discern, to give insight. The Alexandrian Jewish scribes translated into Greek about 200 b.c. the Hebrew text of Proverbs 2:10, “wisdom will enter your heart” by “wisdom will come into your understanding (dianoian [diavnoia])” because to them it meant the same thing. When a person lacks insight the Hebrew speaks of a “lack of heart.”
Understanding cannot be separated from morals. Isaiah was commissioned: “Make the heart of this people calloused; otherwise they might understand with their hearts” ( Isa 6:10 ). Pharaoh hardened his heart lest he hear Moses and gain insight about the Lord ( Exod 8:15 ), and the Lord hardened it irrevocably ( 7:13 ;  9:12 ). Paul says of the perverse, their foolish hearts were darkened ( Rom 1:21 ); they could not see the light of moral truth. The hearts of saints, however, are enlightened ( 2 Col 4:6 ; Eph 1:18 ).
Moderns speak of learning by heart, by which they mean rote memory. In the Bible, however, learning by heart is not like memorizing the multiplication tables; it must be mixed with spiritual affections. The Lord complains of apostate Israel that their worship “is made up only of rules taught by men” but “their hearts are far from me” ( Isa 29:13 ).
As the mouth reveals what is the heart, the ear determines what goes into it. The father tells his son to “store up my commands within you”; he then adds: by “turning your ear to wisdom, and you will incline your heart to understanding” ( Prov 2:2 ). When Moses says, “these commandments are to be upon your hearts” ( Deut 6:6 ), he commands his hearers to remain conscious of them. This idea is expressed by the metaphor of writing on the tablet of the heart ( Prov 3:3 ;  Jer 17:1 ). In short, the heart needs to be educated by filling it with God’s word ( Prov 22:17-18 ). In that way a person will grow in favor and good name ( 3:3-4 ) and be safeguarded against sin ( Psalm 119:11 ).
The heart functions as the conscience. After David showed insubordination against the anointed king by cutting off the corner of his robe, his heart smote him ( 1 Sam 24:5 ), and after Peter’s sermon the audience was “cut to the heart” ( Acts 2:37 ). The heart may condemn us, but God is greater than our hearts ( 1 John 3:20 ). David prays that God would create for him a pure heart to replace his defiled conscience ( Psalm 51:10 ).
Finally, the heart plans, makes commitments, and decides. It is the inner forum where decisions are made after deliberation; here a person engages in self-talk. “In his heart a man plans his course, but the Lord determines his steps” ( Prov 16:9 ). Because of this critical function, the father instructs the son: “Above all else, guard your heart, for it is the wellspring of life ( 4:23 ). The Lord detests “a heart that devises wicked schemes” ( 6:18 ).
The greatest commandment according to Jesus is “Love the Lord your God with all your heart” ( Matt 22:37 ). Love here is more than emotion; it is a conscious commitment to the Lord.
One speaks to the heart of another to move that person to a decision ( Isa 40:2 ;  Hosea 2:14 ). The father asks the son for his heart ( Prov 23:26 ), by which he means that the son make a conscious decision to follow his instructions. The impenitent, however, have hearts that are insensitive, obstinate ( Mark 3:5 ;  6:52 ), and hard ( Matt 19:8 ); they cannot be moved in a new direction.