9-11 Then Moses wrote this law and gave it to the priests, the sons of Levi, who carried the ark of the covenant of the Lord, and to all the elders of Israel. And Moses commanded them, “At the end of every seven years, at the set time in the year of release, at the Feast of Booths,  when all Israel comes to appear before the Lord your God at the place that he will choose, you shall read this law before all Israel in their hearing. 12-13 Assemble the people, men, women, and little ones, and the sojourner within your towns, that they may hear and learn to fear the  Lord  your God, and be careful to do all the words of this law,  and that their children, who have not known it, may hear and learn to fear the Lord your God, as long as you live in the land that you are going over the Jordan to possess.” 14-15  And the Lord said to Moses, “Behold, the days approach when you must die. Call Joshua and present yourselves in the tent of meeting, that I may commission him.” And Moses and Joshua went and presented themselves in the tent of meeting. And the Lord appeared in the tent in a pillar of cloud. And the pillar of cloud stood over the entrance of the tent.

16-17 And the  Lord said to Moses, “Behold, you are about to lie down with your fathers. Then this people will rise and whore after the foreign gods among them in the land that they are entering, and they will forsake me and break my covenant that I have made with them. Then my anger will be kindled against them in that day, and I will forsake them and hide my face from them, and they will be devoured. And many evils and troubles will come upon them, so that they will say in that day, ‘Have not these evils come upon us because our God is not among us?’ 18-19 And I will surely hide my face in that day because of all the evil that they have done, because they have turned to other gods. “Now therefore write this song and teach it to the people of Israel. Put it in their mouths, that this song may be a witness for me against the people of Israel. Deuteronomy 31:9-19 (English Standard Version)

It's often said by those who don't care for Christianity that religions are just myths and stories that were passed down through the ages, much like folk lore and fairy tales. But what do vs 9-11 tell us Moses did?

He wrote down all that God had made known to him. He also instructed that it be read as a whole at least once every seven years so that the next generation might learn to fear God in the right sense of that word, and so that the nation would continue and not fall into paganism. (vs 12-13) The Bible isn't a batch of word of mouth anecdotes passed down from an imaginative grandfather – it is the written record of things God made known to men He chose. The apostle Peter reminded Christians that “no prophecy of Scripture comes from someone's own interpretation - for no prophecy was ever produced by the will of man, but men spoke from God as they were carried along by the Holy Spirit.” (2 Peter 1:20-21)

But the best testimony to the Bible came from Jesus Himself. After rising from the grave He appeared to the disciples, and even ate some food with them, and He said to them: “These are my words that I spoke to you while I was still with you, that everything written about me in the Law of Moses and the Prophets and the Psalms must be fulfilled.” And Luke writes: “Then Jesus opened their minds to understand the Scriptures.” (Luke 24:44-45)

One reason why Moses wrote the laws God gave was because his time on earth was nearly over. (vs 14-15). What was the other reason, a sad one, why Moses wrote down the laws (vs 16-17), and what else did God tell him to do in preparation for Israel's future? (vs 18-19)

God, who knows the hearts of men and women, knew that Israel would be unfaithful to Him and would soon run after the false gods of the nations. The written Law would leave them no excuse for their spiritual adultery. But He took it a step further. Written words can be forgotten by people, but songs often stick in the mind longer. So God tells Moses He would give him a song to teach to the people which would serve as a witness against them in the days when they would turn away from Him. What a contrast this is to what the experience now is for those who truly love the Lord. His word is read and obeyed - and the songs they sing are songs of praise and thanksgiving. Praise God for His written word!