I will give them one heart and put a new spirit in them; I will remove the heart of stone from their bodies and give them a heart of flesh. (Ezek. 11:19)
The Christological interpretation arises from equating the heart of stone with the Law of Moses and the heart of flesh with Jesus, the incarnate Son. This equating may not be legitimate, of course, but surely echoes can be found in New Testament texts such as John 1:17 and Galatians 3:23-24, where (the Law of) Moses and Christ are contrasted.