"The heart is deceitful above all things, and desperately wicked: who can know it?" (Jeremiah 17:9).
Just as in modern languages, ancient Hebrew used the term "heart" to mean the inward motivations which control a person's words and deeds. According to the prophet Jeremiah, the spiritual heart is so innately wicked and deceptive that one cannot even understand his own heart--but God does!
One urgently needs a new heart, and God promises just that. "A new heart also will I give you, and a new spirit will I put within you: and I will take away the stony heart out of your flesh, and I will give you an heart of flesh" (Ezekiel 36:26). "This is the covenant that I will make with them . . . saith the Lord, I will put my laws into their hearts" (Hebrews 10:16).
The problem, of course, is heart attitude. In the third chapter of Hebrews, this is illustrated in terms of the attitudes of the children of Israel in the wilderness. First, their hearts had become hard hearts through their lack of gratitude, and three times the author warns us to "harden not your hearts" (3:8,15; 4:7). As a result, they soon acquired erring hearts. "They do alway err in their heart; and they have not known my ways" (3:10). Finally, their hearts were evil hearts, and God would warn us through them. "Take heed, brethren, lest there be in any of you an evil heart of unbelief, in departing from the living God" (v.12).
An evil heart is defined here, in effect, as an unbelieving heart. A heart which refuses to receive and believe the Word of God is a wicked heart, inevitably generating wicked ways and evil doings. But Christ will create a new heart for all who will believe on Him and confess Him as God and Savior. "That if thou shalt confess with thy mouth the Lord Jesus, and shalt believe in thine heart that God hath raised Him from the dead, thou shalt be saved. For with the heart man believeth unto righteousness" (Romans 10:9-10). HMM