Heart (2588) (kardia) does not refer to the physical organ but is always used figuratively in Scripture to refer to the seat and center of human life. The heart is the center of the personality, and it controls the intellect, emotions, and will. No outward obedience is of the slightest value unless the heart turns to God. Kardia - 156x in 149v - Mt 5:8-note, Mt 5:28-note; Mt 6:21-note; Mt 9:4; 11:29; 12:34, 40; 13:15, 19; 15:8, 18, 19; 18:35; 22:37; 24:48; Mk. 2:6, 8; 3:5; 6:52; 7:6, 19, 21; 8:17; 11:23; 12:30, 33; Lk 1:17, 51, 66; 2:19, 35, 51; 3:15; 5:22; 6:45; 8:12, 15; 9:47; 10:27; 12:34, 45; 16:15; 21:14, 34; 24:25, 32, 38; Jn 12:40; 13:2; 14:1, 27; 16:6, 22; Acts 2:26, 37, 46; 4:32; 5:3, 4; 7:23, 39, 51, 54; 8:21, 22; 11:23; 13:22; 14:17; 15:9; 16:14; 21:13; 28:27; Ro 1:21-note, Ro 1:24-note; Ro 2:5-note, Ro 2:15-note, Ro 2:29-note; Ro 5:5-note; Ro 6:17-note; Ro 8:27-note; Ro 9:2-note; Ro 10:1-note, Ro 10:6-note, Ro 10:8, 9-note, Ro 10:10-note; Ro 16:18-note; 1Co. 2:9; 4:5; 7:37; 14:25; 2Co. 1:22; 2:4; 3:2, 3, 15; 4:6; 5:12; 6:11; 7:3; 8:16; 9:7; Ga 4:6; Ep 1:18-note; Ep 3:17-note; Ep 4:18-note; Ep 5:19-note; Ep 6:5-note, Ep 6:22-note; Php 1:7-note; Php 4:7-note; Col 2:2-note; Col 3:15-note, Col 3:16-note, Col 3:22-note; Col 4:8-note; 1Th 2:4-note, 1Th 2:17-note; 1Th 3:13-note; 2Th 2:17; 3:5; 1Ti 1:5; 2Ti 2:22-note; He 3:8-note, He 3:10-note, He 3:12-note, He 3:15-note; He 4:7-note, He 4:12-note; He 8:10-note; He 10:16-note, He 10:22-note; He 13:9-note; Jas 1:26-note; Jas 3:14; Jas 4:8-note; Jas 5:5, 8; 1Pe 1:22-note; 1Pe 3:4-note, 1Pe 3:15-note; 2Pe 1:19-note; 2Pe 2:14-note; 1Jn 3:19, 20, 21; Re 2:23-note; Re 17:17-note; Re 18:7-note. NAS = kardia as heart, 102; heart's, 1; hearts, 49; mind, 2; minds, 1; quick, 1; spirit, 1. Kardia gives us medical terms such as cardiac, cardiovascular, etc. Just as the integrity of our physical heart is vital to our physical life, in a similar and even more important way the integrity of our spiritual heart is vital to our spiritual life, for our spiritual life impacts not just our enjoyment of time but of eternity. Beloved, let us study the meaning of kardia, but let us be far more serious about guarding our kardia in this short life (see Pr 4:23-note, cp 1Ti 4:7, 8-note, 2Co 5:9-note 2Co 5:10-note, Spurgeon writes "A short life should be wisely spent. We have not enough time at our disposal to justify us in misspending a single quarter of an hour. Neither are we sure of enough life to justify us in procrastinating for a moment. If we were wise in heart we should see this, but mere head wisdom will not guide us aright." See his full note on Ps 90:12-note) Hughes explains that what Paul is saying here is that... "The heart is the wellspring of man’s spiritual life, and that is where the Roman Christians’ obedience was rooted. It was not just a formal obedience—it came from the center of their being. This is the example of slavery Paul holds up for us all: a heartfelt obedience to Christ and his Word. It is an obedience which brings liberation." (Hughes, R. K. Romans: Righteousness from heaven. Preaching the Word. Wheaton, Ill.: Crossway Books) While kardia does represent the inner person, the seat of motives and attitudes, the center of personality, in Scripture it represents much more than emotion, feelings. It also includes the thinking process and particularly the will. For example, in Proverbs we are told, “As (a man) thinks in his heart, so is he” (Proverbs 23:7). Jesus asked a group of scribes, “Why are you thinking evil in your hearts?” (Matthew 9:4). The heart is the control center of mind and will as well as emotion. The great Puritan writer John Flavel wrote that... THE heart of man is his worst part before it is regenerated, and the best afterward; 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 greatest difficulty in conversion, is to win the heart to God; and the greatest difficulty after conversion, is to keep the heart with God. Here lies the very force and stress of religion; here is that which makes the way to life a narrow way, and the gate of heaven a strait gate. (from Proverbs 4:23 Keeping The Heart which has been called "one of greatest Christian books of all time" - Recommended Reading!) The Scottish writer John Eadie says that... The “heart” belongs to the “inner man,” is the organ of perception as well as of emotion; the centre of spiritual as it is physically of animal life. Vine writes that kardia... came to denote man’s entire mental and moral activities, and to stand figuratively for the hidden springs of the personal life, and so here signifies the seat of thought and feeling. (Vine, W. Collected writings of W. E. Vine. Nashville: Thomas Nelson ) MacArthur commenting on kardia writes that... While we often relate heart to the emotions (e.g., “He has a broken heart”), the Bible relates it primarily to the intellect (e.g., “Out of the heart come evil thoughts, murders, adulteries, fornications, thefts, false witness, slanders,” Matt 15:19). That’s why you must “watch over your heart with all diligence” (Proverbs 4:23-note). In a secondary way, however, heart relates to the will and emotions because they are influenced by the intellect. If you are committed to something, it will affect your will, which in turn will affect your emotions." (Drawing Near. Crossway Books) MacArthur adds that "In most modern cultures, the heart is thought of as the seat of emotions and feelings. But most ancients—Hebrews, Greeks, and many others—considered the heart to be the center of knowledge, understanding, thinking, and wisdom. The New Testament also uses it in that way. The heart was considered to be the seat of the mind and will, and it could be taught what the brain could never know. Emotions and feelings were associated with the intestines, or bowels." (MacArthur, J: Ephesians. 1986. Chicago: Moody Press) Marvin Vincent has a lengthy comment on kardia writing... Heart (kardia). The heart is, first, the physical organ, the centre of the circulation of the blood. Hence, the seat and centre of physical life. In the former sense it does not occur in the New Testament. As denoting the vigor and sense of physical life, see Acts 14:17; Jas 5:5; Luke 21:34. It is used fifty-two times by Paul. Never used like psuche soul, to denote the individual subject of personal life, so that it can be exchanged with the personal pronoun (Acts 2:42 Acts 3:21; Romans 13:1 - note); nor like pneuma spirit, to denote the divinely-given principle of life. It is the central seat and organ of the personal life (psuche) of man regarded in and by himself, Hence it is commonly accompanied with the possessive pronouns, my, his, thy, etc. Like our heart it denotes the seat of feeling as contrasted with intelligence. 2Cor 2:4; Ro 9:2 (note); Ro 10:1 (note); 2Cor 6:11; Php 1:7 (note). But it is not limited to this. It is also the seat of mental action, feeling, thinking, willing. It is used — 1. Of intelligence, Romans 1:21 (note) 2Cor 3:15; 4:6; Ep 1:18 (note). 2. Of moral choice, 1Cor 7:37 2Cor 9:7. 3. As giving impulse and character to action, Romans 6:17 (note); Ephesians 6:5 (note); Col 3:22 (note); 1Ti 1:5; 2Timothy 2:22 (note) . The work of the law is written on the heart, Ro 2:15 (note). The Corinthian Church is inscribed as Christ’s epistle on hearts of flesh, 2Cor 1:23. 4. Specially, it is the seat of the divine Spirit, Gal 4:6; Romans 5:5 (note); 2Cor 1:22. It is the sphere of His various operations, directing, comforting, establishing, etc., Php 4:7 (note); Col 3:15 (note); 1Th3:13 (note); 2Th 2:17; 3:5. It is the seat of faith, and the organ of spiritual praise, Ro 10:9 (note); Acts 2:42 Ephesians 5:19 (note); Colossians 3:16 (note). It is equivalent to the inner man, Ep 3:16 (note); Ep 3:17 (note). Its characteristic is being hidden, Ro 2:28, 29 (note); Ro 8:27 (note); 1Cor 4:5; 14:25. It is contrasted with the face, 1Th 2:17 (note); 2Cor 5:12; and with the mouth, Ro 10:8 (note). The phrase "became obedient from the heart" is important. Paul's point is that God works His salvation in a person's innermost being, changing the heart of every sinner who places their trust into Christ's finished work on Calvary. Now in a believer obedience is not forced or legalistic, but is willing. The corollary is that a person whose heart has not been changed and does not possess this desire to obey has not been saved. Robert Haldane adds that... Christian obedience is obedience from the heart, in opposition to an obedience which is by constraint. Any attempt at obedience by an unconverted man, is an obedience produced by some motive of fear, self—interest, or constraint, and not from the heart. Nothing can be more convincing evidence of the truth of the Gospel than the change which, in this respect, it produces on the mind of the believer. Nothing but almighty power could at once transform a man from the love of sin to the love of holiness. (Haldane, R. An Exposition of Romans - Online) (Bolding added)