Meaning
The heart is the "home of the personal life," and hence a man is designated, according to his heart, wise (1Kings 3:12, etc.), pure (Ps 24:4; Matt 5:8, etc.), upright and righteous (Gen 20:5, 6; Ps 11:2; 78:72), pious and good (Luke 8:15), etc. In these and such passages the word "soul" could not be substituted for "heart."
The heart is also the seat of the conscience (Rom 2:15). It is naturally wicked (Gen 8:21), and hence it contaminates the whole life and character (Matt 12:34; 15:18; comp. Eccl 8:11; Ps 73:7). Hence the heart must be changed, regenerated (Ezek 36:26; 11:19; Ps 51:10-14), before a man can willingly obey God.
The process of salvation begins in the heart by the believing reception of the testimony of God, while the rejection of that testimony hardens the heart (Ps 95:8; Prov 28:14; 2Chr 36:13). "Hardness of heart evidences itself by light views of sin; partial acknowledgment and confession of it; pride and conceit; ingratitude; unconcern about the word and ordinances of God; inattention to divine providences; stifling convictions of conscience; shunning reproof; presumption, and general ignorance of divine things."