Popular advice often enjoins us follow our heart. In the Scriptures, the heart often refers to one’s inner person encompassing our thoughts, feelings, and volition. The desire of our heart may be reliable (Psalm 37:4) or our heart may lead us astray (Deuteronomy 30:17). The prophet Jeremiah gave a stern warning to the people of Judah:

The sin of Judah is written with an iron stylus; with a diamond point it is engraved on the tablet of their hearts and on the horns of their altars, (Jeremiah 17:1)

The people had sinned egregiously and followed after other gods. They were trusting in the desires of their hearts rather than trusting in God:

This is what the Lord says: “Cursed is the man who trusts in mankind and makes flesh his strength, and whose heart turns away from the Lord.” (Jeremiah 17:5)

Jeremiah then described the seriousness of the condition of the human heart:

The heart is more deceitful than all else and is desperately sick; who can understand it? (Jeremiah 17:9)

The word deceitful in its original sense means steep and hilly and by implication it conveys the idea of being deceitful, sly, and insidious. The phrase desperately sick is a Hebrew word that is used several times in the book of Jeremiah and is translated as being incurable or irreparable. God does not patch up our old heart, He gives us a new heart. Ezekiel taught that God can take away our hard heart and give us a new heart:

Moreover, I will give you a new heart and put a new spirit within you; and I will remove the heart of stone from your flesh and give you a heart of flesh. (Ezekiel 36:26)

Although God may give us a new heart, we can still choose to follow after our fleshly desires. The Apostle Paul clarified this distinction as he described the battle between the flesh and the Spirit. Paul knew that he was a new creation in Christ and Christ had given him the victory, but he still battled his fleshly desires:

Thanks be to God through Jesus Christ our Lord! So then, on the one hand I myself with my mind am serving the law of God, but on the other, with my flesh the law of sin. (Romans 7:25)

Although believers have a new heart, they must bring their minds and actions into conformity with their new identity. Justification is punctiliar; but sanctification is a process. Our sanctification is only completed in our glorification, and that is yet to come. God knows the thoughts and intentions of our hearts and we have a responsibility to walk by the Spirit so that we will not carry out the desires of the flesh:

I, the Lord, search the heart, I test the mind, to give to each person according to his ways, according to the results of his deeds. (Jeremiah 17:10)