Moses lays before Israel the will of God in plain terms: fear the LORD, walk in His ways, love Him, serve Him with all your heart and soul, and keep His commandments and statutes — all for their own good. Being in harmony with God, the sermon argues, is nothing more mystical than being in agreement with Him about His will and holy purpose; it is not a vague feeling but a concrete commitment to His revealed law and covenant.
Moses sets that agreement as a choice: life and prosperity, or death and adversity, blessing or curse, with heaven and earth called as witnesses. The New Testament fulfills rather than cancels this: Christ has redeemed the firstfruits of a fallen world for Himself, and as believers walk according to His law, they are being further conformed to His will and holy purposes — true harmony restored.