Even though Moses wouldn’t live to see it, he’s still giving them instructions on what to do the moment they cross over the Jordan into Canaan.
Step one on that list is to take a few enormous stones and copy the law on them.
This wasn’t a unique idea. It was common in many cultures in those days to write on plaster, which, in an arid environment, such as Egypt or the Sinai Peninsula, would allow the words to last a really long time.
Nor was it a unique idea to write the law down. Kings were told to copy the law by hand as soon as they ascended to the throne (Deuteronomy 17:18-20), and then keep that version close by as a guide for their reign.
Nor is writing them on stones. The Ten Commandments were written on stone tablets, after all, and many cultures wrote covenants on stone, such as Egypt, Greece, and Persia. Although written on clay and not stone, the code of Ur-Nammu is one of the oldest known examples.
(As a side point, it seems like the stones that they were to write on were different from the stones used for the altar in Deuteronomy 27:5-7. That was a regular altar made of uncut stones, whereas these stones were used to hold the Law. Two different topics.)
So what makes this moment unique? The fact that the law is between man and God.
In every other form of stone-written law code that I could find, whatever deity existed for them was largely in the background. They were a guarantor, or even a law-giver, but never a covenant-maker. They never made “agreements” with man that bound both parties, in other words.
That’s part of what makes this law on the stones so special. Not only are we talking about an all-powerful God that does miracles through the desert for His people, but one in which He willingly (and benevolently) binds Himself to an agreement with them.
And writing the Law on stone cements this in place.
However, logistical objections immediately erupt over two things. First, how much of the law were they to copy, and, depending on the amount, how on earth could they find stones big enough to fit what they needed to write?
On the first point, many scholars have offered a host of ideas, saying that God only intended the Decalogue, while others contend that it was the entire book of Deuteronomy.
To the second point, if God indeed did intend for the entire Law to be written on stones, it’s not without precedent. The entire code of Hammurabi is currently inscribed on a single stone that now sits in the Louvre in Paris. While it would require more stones (because Deuteronomy is longer), it’s still at least feasible.
Let’s not lose sight of the main idea here, though: The point was that God wanted these codes to not only serve as a reminder but also as a witness to these people.
Moreover, God said that these words should be written “very distinctly” (Deuteronomy 27:8). Some traditions argue that that means the Law was copied in seventy different languages, so that there was no excuse for the people to not know what God wanted.
If that’s the case, the point here is clear: Know the Law, follow the Law. And live.