Jeremiah 17:9 is a well-known verse that, unfortunately, is frequently misunderstood and often misapplied.
It reads: “The heart is deceitful above all things, and desperately sick; who can understand it?” True enough. The problem arises when Christian people apply it to themselves. You sometimes hear it in Christian songs, read it in Christian counseling books, even find it in classic Christian devotional writings—frequently from sources that are otherwise very reliable. Even my pastoral hero, John Newton, once wrote in a letter about how “my evil heart obstructs my judgment.”
The problem with this view is that it no longer applies to Christians, who have been regenerated by God’s Spirit. Per the explicit terms of the New Covenant, Christians have a brand new, pure, sinless, submissive, righteous, law-keeping, sincere, good heart!
Remember how God puts it later in Jeremiah? “This is the covenant that I will make with the house of Israel after those days, declares the LORD: I will put my law within them, and I will write it on their hearts” (31:33).
He’s even more explicit in Ezekiel: “I will give them one heart, and a new spirit I will put within them. I will remove the heart of stone from their flesh and give them a heart of flesh, that they may walk in my statutes and keep my rules and obey them” (11:19-20; cf. 36:26).
This is consistent with the rest of the Bible’s testimony that Christianity does not simply give you something new; it makes you something new. “Therefore, if anyone is in Christ, he is a new creation. The old has passed away; behold, the new has come” (2 Cor 5:17). “You have put off the old self with its practices and have put on the new self, which is being renewed in knowledge after the image of its Creator” (Col 3:9-10).
None of this is to say Christians are sinlessly perfect. It’s simply to say the old you is dead, and you really, truly are new! You can say with Paul: “I have been crucified with Christ. It is no longer I who live, but Christ who lives in me” (Gal 2:20).
As a youth, Augustine lived a promiscuous, pagan lifestyle before he was converted. Sometime after his conversion, he was seen by a former girlfriend who cried out to get his attention, “Augustine, it is I!” “Yes,” he called back earnestly, “but it is not I.”