He says if the New Covenant is to be better than the Old, it has to provide for man's obedience. "The New Covenant provides a guarantee, not only for God’s faithfulness, but for man’s too! And this in no other way than by God Himself undertaking to secure man’s part as well as His own."
This reminds me of what I'd thought and written about the New Covenant in Covenant of Hope. The old covenant failed because we couldn't keep it. "Do these things and you will live" is not enough. We need God to do in us what we cannot do in ourselves.