“‘Heart’ is used different ways in the Bible. Sometimes it is the mind believing or being enlightened (Rom. 10:10; Eph. 1:18), the will deciding and acting (2 Cor. 9:7, Eph. 6:6), or the affections feeling (2 Cor. 2:4). In Hebrews 4:12 the heart thinks and feels. The best way to think of your heart is that it comprises
-Your thoughts, plans judgments, discernment (the mind);
-Your choices and actions (the will);
-Your longings, desire, revulsion, imagination, feelings (the affections);
-Your sense of right and wrong, which approves or condemns your mind, will, and affections (the conscience).
This summary is based on John Owen’s discussion of the same in Indwelling Sin and The Mortification of Sin. Consider the above and then consider how the Bible says the heart by nature (apart from Christ) is deceitful above all things (Jer. 17:9). Also consider what it means when the Holy Spirit graciously gives sinners new hearts (Ezek. 36:26). Check out chapter three of The Enemy Within by Kris Lundgaard for more info on this theme of the heart.