Synopsis of the Books of the Bible, by John Nelson Darby, [1857-62], at sacred-texts.com
We must now dwell a little on this last point. Chapter 30 furnishes us with an important principle. It supposes that the people have already incurred the consequences of disobedience, and they are seen as driven out of the land, and strangers among distant nations. The law could not be followed out in such a case; and, indeed, the violation of the law had even then produced its fruits. But then quite a new principle is introduced: the return of the hearts of the people to Jehovah, and obedience, one must add, in spirit. Thereupon Jehovah brings them back into their land, and blesses them in it. The curse is put on their enemies; and they are to observe in the land the ordinances of Jehovah, enjoying anew His full blessing; for the commandment was neither in heaven, nor beyond the seas, but in the mouth and in the heart. This was not the new covenant, but faith laying hold of the spirit of the law in principle, and turning the heart towards Jehovah, when the law was externally impracticable.
The establishment of the new covenant, based on this return of the heart, at a time appointed of God, will be something well defined. Here we have the principle of their return when under the curse of the law they had broken. Hence, the apostle quotes this passage for the basis of the principle, as a testimony given to what righteousness by faith was, applying it to Christ Himself-the return of the heart to the object and end of the law, when judgment was on them for its violation, and hope of righteousness by its accomplishment impossible-how Christ was the end of the law for righteousness. The principle is found here. The apostle brings in Christ as the true accomplishment of it. At the end of the chapter, Moses declares that he has now set before them the good and the evil, and that they would have to bear the consequence of their choice.