According to the Bible, the heart is the centre not only of spiritual activity, but of all the operations of human life. "Heart" and "soul" are often used interchangeably (Deuteronomy 6:5; 26:16; Comp. Matthew 22:37; Mark 12:30, 33), but this is not generally the case.
The heart is the "home of the personal life," and hence a man is designated, according to his heart, wise (1 Kings 3:12, etc.), pure (Psalm 24:4; Matthew 5:8, etc.), upright and righteous (Genesis 20:5, 6; Psalm 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 (Romans 2:15). It is naturally wicked (Genesis 8:21), and hence it contaminates the whole life and character (Matthew 12:34; 15:18; Comp. Ecclesiastes 8:11; Psalm 73:7). Hence the heart must be changed, regenerated (Ezek. 36:26; 11:19; Psalm 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 (Psalm 95:8; Proverbs 28:14; 2 Chronicles 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."