Scripture insists that fallen man needs an entirely new start to walk with God. It's not enough to have forgiveness of sins, nor justification, nor redemption, nor even atonement. Israel under Law had all these, but it wasn't enough. Isaiah 5 describes God as the owner of a vineyard, looking for fruit in it. Finding none, He asks, "What more could I have done?" (Isaiah 5:4). The fruit God was looking for from Israel needed something more. It needed a new heart (Ezekiel 36:26).
Israel's history under the Law was an experiment demonstrating that fallen man can't please God (Romans 3:20). God gave the nation His Law with the promise that if they kept it they would be blessed (Exodus 19:1–6). By the time they got from Sinai to the Jordan, they had proven they could not. Darby comments on the beginning of Deuteronomy:
That which strikes one in the first chapters is, the pains that Jehovah takes to present all possible motives to that poor people to lead them to obedience, in order that they may be blessed. These things, which ought at least to have touched the heart, served, alas! only to prove its hardness, and to shew that, if man is to be blessed, God must give him a new heart, as it is written in the chapter which closes the second part of His exhortations to obedience: "Yet Jehovah hath not given you a heart to perceive, and eyes to see, and ears to hear, unto this day" (chap. 29: 4). (Synopsis, Volume 1)
When we come to the time of the Babylonian Captivity, the prophets are declaring that God will give a New Covenant. Jeremiah describes the New Covenant in Jeremiah 31:31–40. He says it will be "not according to the covenant that I made with their fathers, in the day of my taking them by the hand, to lead them out of the land of Egypt; which my covenant they broke" (Jeremiah 31:32). Ezekiel's description of the New Covenant (Ezekiel 36:24–38; Ezekiel 37:15–28) hinges on God giving the people "a new heart" (Ezekiel 36:26), Jeremiah talks about God writing His laws on their hearts (Jeremiah 31:33).
(Interestingly, Jeremiah 31, Ezekiel 37, and Hosea 14 all say the New Covenant will be made when God brings back Ephraim and the ten tribes, restoring all twelve tribes of Israel back into a single nation.)
So by the end of the Old Testament, it's plain from Israel's history under Law that we need a new heart to obey God. Fallen man – even justified, forgiven, delivered, instructed, and rebuked fallen man – can't do it. There needs to be a new creation.
When the Lord Jesus speaks with Nicodemus, He insists a man can't see God's kingdom unless he's been born again (John 3:3). When Nicodemus cavils on this point, the Lord Jesus tells him a ruler of Israel ought to know that a new birth is needed (John 3:10). Why does the Lord Jesus say a ruler of Israel ought to know this already? Because the Old Testament explicitly teaches the need of a new birth: that's the whole point of Deuteronomy 29:4 and Ezekiel 36:26.
Every single command in the New Testament must be understood in light of this: God is not interested in the "good works" done from an unregenerate heart. God's not impressed by unregenerate men and women, even if they're good. Every single command in the New Testament assumes new creation as the starting point.
There's more to it, of course; but this is the starting point. Have you been born again?
(Since I mentioned Isaiah 5, let me recommend a message by H. E. Hayhoe, "Key to the Old Testament". He says Isaiah 5 is the key to understanding the whole Old Testament. That message is worth your time.)