Why did God promise to give a new heart of flesh to His Old Testament people?

Our natural condition is to have a heart of stone, which means we’re dead to the things of God. When we’re reborn by the Holy Spirit, that which was stony now is flesh—it’s alive. That’s problematic because in the New Testament the term “flesh” is a synonym often for our sin nature. So, God is not saying in the Old Testament, “Someday I’m going to give you a sin nature”—you already have that—but rather He’s saying that He will change the disposition of the heart. The Bible makes it clear that people in the Old Testament were born again by the power of the Holy Spirit, just as in the New Testament. This is what Paul labors in Romans 3 and 4, where he presents Abraham as the model of what it means to be both justified and a Christian. Abraham was renewed inwardly by the power of the Holy Spirit, who changed his heart from a heart of stone to a heart of flesh. Most of Israel remained stone-hearted in the history of the Old Testament. They became stiff-necked and hardhearted. So, God promised a future outpouring of His Spirit where there would be multitudes of people, not just Jews but Gentiles as well, who would be born of the Spirit.