Deuteronomy 31:27 KJV
For I know thy rebellion, and thy stiff neck: behold, while I am yet alive with you this day, ye have been rebellious against the LORD; and how much more after my death?
Deuteronomy 31:29 KJV
For I know that after my death ye will utterly corrupt yourselves, and turn aside from the way which I have commanded you; and evil will befall you in the latter days; because ye will do evil in the sight of the LORD, to provoke him to anger through the work of your hands.
Moses knew the hearts of the people to be both rebellious and of a corrupt nature. And no wonder.
First, he had the experience of 40 years.
• He had seen their rebellion.
• He had heard their murmuring.
• He had witnessed how quickly they could be turned aside to other gods.
Secondly Moses knew his own heart.
While he had been a humble and meek servant of the Lord, still he would not personally enter into the Promised Land because of his unbelief.
Now, against this rebellious and corrupt nature; God placed the Law. It was "a witness against thee".
God's law both highlights our nature as sinful and points us to a way of life. Covered as it was, in that ark, the people were forever reminded that blood atoned for the breaking of the Law, which is sin. Christ's blood shed at Calvary forever atones for that sin and provides access into the holiest place with God.