Jeremiah 17:9 says, “The heart is deceitful above all things, and desperately wicked; who can know it?” The heart is the seat of the affections. It is the mind, the understanding. Man, in his sinful state has a heart, that is, a mind and understanding that is deceitful (or crooked, polluted) and is desperately wicked (or incurably sick). The problem is that in this state man cannot appear before God for it says in the Psalms, “Who may ascend into the hill of the Lord? Or who may stand in His holy place? He who has clean hands and a pure heart, who has not lifted up his soul to an idol…” (Psalms 24:3-4). Also Jesus said in Matthew 5:8: “Blessed are the pure in heart, for they shall see God.” But apart from this pure heart we cannot. Thus with a wicked and depraved heart we cannot have fellowship with God.
But God promised to give His people a new heart. He says through Ezekiel, “I will give you a new heart and put a new spirit within you; I will take the heart of stone out of your flesh and give you a heart of flesh. I will put My Spirit within you and cause you to walk in My statutes, and you will keep My judgments and do them.” (Ezek 36:26-27) God gives us a new heart through the atoning work of Christ and concerning our salvation Paul wrote, “For he is not a Jew who is one outwardly, nor is circumcision that which is outward in the flesh; but he is a Jew who is one inwardly; and circumcision is that of the heart…” (Romans 2:28-29) Our hearts have been cleansed, purified and made pure. “Therefore,” Paul writes, “if anyone is in Christ, he is a new creation; old things have passed away; behold, all things have become new.” (2 Cor 5:17) In Christ we have been made new. The depraved, incurable heart that we had was crucified with Christ and it no longer lives, but Christ lives within us (Gal. 2:20).
However, lest we become confident in ourselves we must remember that although we are pure before God the old man is still lurking and still trying to influence us. Proverbs warns us that “He who trusts in his own heart (mind or understanding) is a fool, but whoever walks wisely will be delivered.” (Prov. 28:26) We are to trust the Word of God above all and test everything against it, including our own reasoning and thoughts. But praise the Lord for the new heart He has given us, a heart that is soft towards Him, a heart of faith and love. It is not our heart, not our mind; "...we have the mind of Christ" (1 Cor. 2:16).