The Hebrew phrase translated as "too hard" is לֹֽא־נִפְלֵ֥את, the "hard" part being from the root פלא, which often has the meaning of "marvelous." Its common translations are "wonderful, wondrous, marvelous." The Jewish scholar, Robert Alter, translates it here as "wondrous" and comments, "The force of the Hebrew root p-l-' [פלא] is something hidden...or beyond human ken." It is not some esoteric teaching hidden in a heavenly realm or a foreign mystery to which we must gain access by traveling over the seas.
So my reading of that text is that Moses is basically giving them no excuse. God's law is quite clear. Black-and-white shalts and shalt nots, not some esoteric, foreign mystery in an unknown language. It's in your mouth as you utter the words. It is in your heart. So do it. Keep it.
That neither means that we can or shall. It just means that we MUST. The law offers no excuse. It says, "Do this. Don't do this." No loopholes. No mercy. No exceptions.
That is precisely why there is NO hope in the law. No matter how much effort we put into it, how many prayers we offer, how sincere and zealous we are, we cannot love God perfectly nor our neighbor perfectly, which is the sum of the law. We fail both vertically and horizontally.
Therefore, what good news, what BEST of news, is the Gospel, that demands nothing of us but gives us, gratis, the finished work of Christ.
P.S. For Paul's adaptation of Deut. 30 in application to the Gospel, see Romans 10:5-13.
Written by Chad Bird, Scholar-in-Residence