Many sermons and books talk about the “new covenant” of grace that Jesus brings in contrast to the “old covenant” of legalism in the Old Testament. However, that concept is a complete misunderstanding of the Old Testament, especially of the “new covenant” mentioned in Jeremiah 31. There God promises to make a new covenant, but it is with Israel! And the reason a new covenant is needed is that the people have broken it and turned to disobedience (in Old Testament times) or legalism (in Jesus’ day), not because the covenant itself was legalistic. In fact, all the elements of the “new covenant” were there in every previous covenant God made with His people. These elements are a part of the everlasting covenant, which is the plan of salvation in totality. God will be their God, and they will be His people; God will write His law in their hearts; God will forgive their sins; and everyone will know God. Abraham had each of these elements in his covenant with God; so did David! And the Sinai covenant also contained each one, and now as the covenant is renewed here in Deuteronomy, all of those elements are present once again.
So it seems that the main reason that people see the new covenant as a New Testament thing is that they don’t actually read the Old Testament, or they only read parts of it with preconceived notions about what God is like there. Here in Deuteronomy 30, God has compassion on His people and forgives their sins, bringing them back from exile. Moses makes clear that the people need a heart circumcision (Deut. 10:16), but they cannot do it on their own, so God Himself will do it (Deut. 30:6), similar to the way He gives a new heart to His followers in Ezekiel 36. This new heart will enable His people to love Him with all their hearts! This is not legalism but grace and relationship! Love leads to obedience, and God rejoices in us as His people when we return to Him with all our hearts and choose to be His people. In addition, each person will know God personally, since He is our life (v. 20). We are to stick to Him like honey, and He will never let us go. All the elements of the “new covenant” are here in this chapter, as also through the rest of Deuteronomy. Unfortunately, the Israelites continued to break the covenant and reject God, turning to their own works to save them, shutting their hearts to God, refusing to get to know God, and rejecting Him as their God. This is why the new covenant was necessary, as a renewed covenant after the people had broken it again and again.